Panic (slice indexes) when encoding with brotli

We just started getting that last night:

```
panic: slice index starts at 15827 but ends at 8192

Thread 0 Crashed:
0 core 0x558771457352 core::slice::slice_index_order_fail (mod.rs:2758)
1 core 0x558770cec47b [inlined] core::ops::range::Range::index (mod.rs:2917)
2 core 0x558770cec47b [inlined] core::ops::range::RangeFrom::index (mod.rs:2996)
3 core 0x558770cec47b [inlined] core::slice::::index (mod.rs:2732)
4 brotli 0x558770cec47b brotli::enc::compress_fragment::BrotliCompressFragmentFastImpl (compress_fragment.rs:979)
5 brotli 0x558770ceafe1 brotli::enc::compress_fragment::BrotliCompressFragmentFast
6 brotli 0x558770d36dee brotli::enc::encode::BrotliEncoderCompressStreamFast (encode.rs:2762)
7 async_compression 0x558770c7ec25 [inlined] async_compression::codec::brotli::encoder::BrotliEncoder::encode (encoder.rs:39)
8 async_compression 0x558770c7ec25 async_compression::codec::brotli::encoder::BrotliEncoder::encode (encoder.rs:68)
9 async_compression 0x558770cefa53 [inlined] async_compression::stream::generic::encoder::Encoder::poll_next::{{closure}} (encoder.rs:93)
10 async_compression 0x558770cefa53 async_compression::stream::generic::encoder::Encoder::poll_next (encoder.rs:73)
11 async_compression 0x558770cecca6 [inlined] async_compression::stream::BrotliEncoder::poll_next (encoder.rs:64)
12 futures_core 0x558770cecca6 [inlined] futures_core::stream::TryStream::try_poll_next (stream.rs:193)
13 futures_util 0x558770cecca6 [inlined] futures_util::stream::try_stream::into_stream::IntoStream::poll_next (into_stream.rs:40)
14 futures_util 0x558770cecca6 [inlined] futures_util::stream::stream::map::Map::poll_next (map.rs:61)
15 futures_util 0x558770cecca6 [inlined] futures_util::stream::try_stream::MapOk::poll_next (lib.rs:118)
16 futures_core 0x558770cecca6 [inlined] futures_core::stream::TryStream::try_poll_next (stream.rs:193)
17 futures_util 0x558770cecca6 [inlined] futures_util::stream::try_stream::into_stream::IntoStream::poll_next (into_stream.rs:40)
18 futures_util 0x558770cecca6 futures_util::stream::stream::map::Map::poll_next (map.rs:61)
19 futures_util 0x558770db5419 futures_util::stream::try_stream::MapErr::poll_next (lib.rs:118)
20 hyper 0x5587712b128a hyper::body::body::Body::poll_inner (body.rs:291)
21 hyper 0x5587712b15a7 [inlined] hyper::body::body::Body::poll_eof (body.rs:253)
22 hyper 0x5587712b15a7 hyper::body::body::Body::poll_data (body.rs:323)
23 fly_proxy 0x558770db3f54 fly_proxy::connect::body::LoadGuardedBody::poll_data (body.rs:168)
24 hyper 0x5587709357f5 hyper::proto::h2::PipeToSendStream::poll (mod.rs:162)
25 hyper 0x558770adba20 [inlined] hyper::proto::h2::server::H2Stream::poll2 (server.rs:419)
26 hyper 0x558770adba20 hyper::proto::h2::server::H2Stream::poll (server.rs:437)
27 tokio 0x558770b3f589 [inlined] tokio::util::trace::Instrumented::poll (trace.rs:26)
28 tokio 0x558770b3f589 [inlined] tokio::runtime::task::core::Core::poll::{{closure}} (core.rs:173)
29 tokio 0x558770b3f589 tokio::loom::std::unsafe_cell::UnsafeCell::with_mut (unsafe_cell.rs:14)
30 tokio 0x558770b123f3 [inlined] tokio::runtime::task::core::Core::poll (core.rs:158)
31 tokio 0x558770b123f3 [inlined] tokio::runtime::task::harness::Harness::poll::{{closure}} (harness.rs:107)
32 core 0x558770b123f3 [inlined] core::ops::function::FnOnce::call_once (function.rs:232)
33 std 0x558770b123f3 std::panic::AssertUnwindSafe::call_once (panic.rs:318)
34 std 0x5587708879df [inlined] std::panicking::try::do_call (panicking.rs:297)
35 std 0x5587708879df [inlined] std::panicking::try (panicking.rs:274)
36 std 0x5587708879df [inlined] std::panic::catch_unwind (panic.rs:394)
37 tokio 0x5587708879df tokio::runtime::task::harness::Harness::poll (harness.rs:89)
38 tokio 0x558771310bf0 [inlined] tokio::runtime::task::raw::RawTask::poll (raw.rs:66)
39 tokio 0x558771310bf0 [inlined] tokio::runtime::task::Notified::run (mod.rs:169)
40 tokio 0x558771310bf0 [inlined] tokio::runtime::thread_pool::worker::Context::run_task::{{closure}} (worker.rs:374)
41 tokio 0x558771310bf0 [inlined] tokio::coop::with_budget::{{closure}} (coop.rs:127)
42 std 0x558771310bf0 [inlined] std::thread::local::LocalKey::try_with (local.rs:263)
43 std 0x558771310bf0 std::thread::local::LocalKey::with (local.rs:239)
44 tokio 0x5587713315f2 [inlined] tokio::coop::with_budget (coop.rs:120)
45 tokio 0x5587713315f2 [inlined] tokio::coop::budget (coop.rs:96)
46 tokio 0x5587713315f2 tokio::runtime::thread_pool::worker::Context::run_task (worker.rs:352)
47 tokio 0x558771330fbf tokio::runtime::thread_pool::worker::Context::run (worker.rs:331)
48 tokio 0x55877131f733 [inlined] tokio::runtime::thread_pool::worker::run::{{closure}} (worker.rs:309)
49 tokio 0x55877131f733 tokio::macros::scoped_tls::ScopedKey::set (scoped_tls.rs:63)
50 tokio 0x5587713307f6 tokio::runtime::thread_pool::worker::run (worker.rs:306)
51 tokio 0x5587713248d0 [inlined] tokio::runtime::thread_pool::worker::Launch::launch::{{closure}} (worker.rs:285)
52 tokio 0x5587713248d0 [inlined] tokio::runtime::blocking::task::BlockingTask::poll (task.rs:41)
53 tokio 0x5587713248d0 [inlined] tokio::runtime::task::core::Core::poll::{{closure}} (core.rs:173)
54 tokio 0x5587713248d0 tokio::loom::std::unsafe_cell::UnsafeCell::with_mut (unsafe_cell.rs:14)
55 tokio 0x5587713343f0 [inlined] tokio::runtime::task::core::Core::poll (core.rs:158)
56 tokio 0x5587713343f0 [inlined] tokio::runtime::task::harness::Harness::poll::{{closure}} (harness.rs:107)
57 core 0x5587713343f0 [inlined] core::ops::function::FnOnce::call_once (function.rs:232)
58 std 0x5587713343f0 std::panic::AssertUnwindSafe::call_once (panic.rs:318)
59 std 0x558771323073 [inlined] std::panicking::try::do_call (panicking.rs:297)
60 std 0x558771323073 [inlined] std::panicking::try (panicking.rs:274)
61 std 0x558771323073 [inlined] std::panic::catch_unwind (panic.rs:394)
62 tokio 0x558771323073 tokio::runtime::task::harness::Harness::poll (harness.rs:89)
63 tokio 0x558771313bc8 [inlined] tokio::runtime::task::raw::RawTask::poll (raw.rs:66)
64 tokio 0x558771313bc8 [inlined] tokio::runtime::task::Notified::run (mod.rs:169)
65 tokio 0x558771313bc8 tokio::runtime::blocking::pool::Inner::run (pool.rs:230)
66 tokio 0x5587713199fd [inlined] tokio::runtime::blocking::pool::Spawner::spawn_thread::{{closure}}::{{closure}} (pool.rs:210)
67 tokio 0x5587713199fd tokio::runtime::context::enter (context.rs:72)
68 tokio 0x55877131c4a6 [inlined] tokio::runtime::handle::Handle::enter (handle.rs:76)
69 tokio 0x55877131c4a6 [inlined] tokio::runtime::blocking::pool::Spawner::spawn_thread::{{closure}} (pool.rs:209)
70 std 0x55877131c4a6 std::sys_common::backtrace::__rust_begin_short_backtrace (backtrace.rs:130)
71 std 0x5587713266bb [inlined] std::thread::Builder::spawn_unchecked::{{closure}}::{{closure}} (mod.rs:475)
72 std 0x5587713266bb [inlined] std::panic::AssertUnwindSafe::call_once (panic.rs:318)
73 std 0x5587713266bb [inlined] std::panicking::try::do_call (panicking.rs:297)
74 std 0x5587713266bb [inlined] std::panicking::try (panicking.rs:274)
75 std 0x5587713266bb [inlined] std::panic::catch_unwind (panic.rs:394)
76 std 0x5587713266bb [inlined] std::thread::Builder::spawn_unchecked::{{closure}} (mod.rs:474)
77 core 0x5587713266bb core::ops::function::FnOnce::call_once{{vtable.shim}} (function.rs:232)
78 alloc 0x558771433d4a [inlined] alloc::boxed::Box::call_once (boxed.rs:1076)
79 alloc 0x558771433d4a [inlined] alloc::boxed::Box::call_once (boxed.rs:1076)
80 std 0x558771433d4a std::sys::unix::thread::Thread::new::thread_start (thread.rs:87)
81 0x7f070b8dc6db start_thread
82 0x7f070b3eda3f __clone
83 0x0
```

I don't have much details about the actual body that's causing these issues. We're running a reverse-proxy with varying responses and workloads.