crash due to absurd memory allocation

Some config file generate an extreme memory allocation when read, that leads to a crash.

Example program that generates the error:

use config::{Config, File, FileFormat};

fn main() {
    let s = vec![0x70, 0x4d, 0x5b, 0x32, 0x32, 0x32, 0x32, 0x32, 0x32, 0x32, 0x32, 0x32,
                 0x32, 0x32, 0x32, 0x32, 0x32, 0x32, 0x32, 0x32, 0x5d, 0x3a, 0x00, 0x00,
                 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x08, 0x33, 0x03, 0x00,
                 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00];

    let _ = Config::new().merge(File::from_str(std::str::from_utf8(&s).unwrap(), FileFormat::Ini));
}

Output:

$ RUST_BACKTRACE=1 ./target/debug/config-reproduce 
memory allocation of 1777777777777777840 bytes failed
Aborted (core dumped)

Stacktrace:

(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:49
#1  0x00007ffff7dc8864 in __GI_abort () at abort.c:79
#2  0x0000555555747e67 in std::sys::unix::abort_internal () at library/std/src/sys/unix/mod.rs:255
#3  0x0000555555581476 in std::process::abort () at library/std/src/process.rs:1957
#4  0x000055555574440e in std::alloc::rust_oom () at library/std/src/alloc.rs:330
#5  0x000055555575bb67 in alloc::alloc::__alloc_error_handler::__rg_oom () at library/alloc/src/alloc.rs:398
#6  0x0000555555582237 in alloc::alloc::handle_alloc_error () at library/alloc/src/alloc.rs:367
#7  0x00005555555fafdc in alloc::raw_vec::handle_reserve (result=...) at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/alloc/src/raw_vec.rs:531
#8  0x00005555555fbb11 in alloc::raw_vec::RawVec<T,A>::reserve::do_reserve_and_handle (slf=0x5555557f2bb0, len=0, additional=22222222222222223) at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/alloc/src/raw_vec.rs:333
#9  0x00005555555fbacb in alloc::raw_vec::RawVec<T,A>::reserve (self=0x5555557f2bb0, len=0, additional=22222222222222223) at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/alloc/src/raw_vec.rs:337
#10 0x0000555555602cd1 in alloc::vec::Vec<T,A>::reserve (self=0x5555557f2bb0, additional=22222222222222223) at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/alloc/src/vec/mod.rs:803
#11 0x0000555555602492 in alloc::vec::Vec<T,A>::extend_with (self=0x5555557f2bb0, n=22222222222222223, value=...) at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/alloc/src/vec/mod.rs:2226
#12 0x0000555555602c66 in alloc::vec::Vec<T,A>::resize (self=0x5555557f2bb0, new_len=22222222222222223, value=...) at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/alloc/src/vec/mod.rs:2122
#13 0x00005555555ee99d in config::path::Expression::set (self=0x7fffffffca10, root=0x7fffffffcbb8, value=...) at /home/capitol/.cargo/registry/src/github.com-1ecc6299db9ec823/config-0.11.0/src/path/mod.rs:245
#14 0x00005555555848f6 in config::source::Source::collect_to (self=0x5555557f1e10, cache=0x7fffffffcbb8) at /home/capitol/.cargo/registry/src/github.com-1ecc6299db9ec823/config-0.11.0/src/source.rs:27
#15 0x0000555555603af0 in <alloc::vec::Vec<alloc::boxed::Box<dyn config::source::Source+core::marker::Send+core::marker::Sync>> as config::source::Source>::collect (self=0x7fffffffdae0)
    at /home/capitol/.cargo/registry/src/github.com-1ecc6299db9ec823/config-0.11.0/src/source.rs:53
#16 0x000055555560357c in config::source::Source::collect_to (self=0x7fffffffdae0, cache=0x7fffffffd380) at /home/capitol/.cargo/registry/src/github.com-1ecc6299db9ec823/config-0.11.0/src/source.rs:17
#17 0x00005555555f9e20 in config::config::Config::refresh (self=0x7fffffffda80) at /home/capitol/.cargo/registry/src/github.com-1ecc6299db9ec823/config-0.11.0/src/config.rs:124
#18 0x0000555555583f16 in config::config::Config::merge (self=0x7fffffffda80, source=...) at /home/capitol/.cargo/registry/src/github.com-1ecc6299db9ec823/config-0.11.0/src/config.rs:78
#19 0x0000555555583a53 in config_reproduce::main () at src/main.rs:12

Start with the reproduced INI input and the allocation path in src/path/mod.rs, especially Expression::set, then trace how it is reached through source.rs. Add a regression test for this input and verify that parsing no longer aborts with an absurd allocation.