fuzz: zenwebp::demux_container crash (d7bab0143d47c925) — RECURRED after https://github.com/imazen/zenwebp/issues/80

Found by the continuous fuzz farm (libFuzzer).

**This signature was previously filed and closed as fixed at https://github.com/imazen/zenwebp/issues/80 — crashes with the identical panic location kept arriving afterward, so this is either an incomplete fix or a re-break, not a duplicate.**

- **Crate**: `zenwebp`
- **Target**: `demux_container`
- **First seen on arch**: `x86_64`
- **Signature** (`d7bab0143d47c925`): `panic zenwebp/src/mux/demux.rs:360:25`
- **Repro + metadata in R2**: `s3://zenfuzz/crashes/zenwebp/demux_container/x86_64/d7bab0143d47c925/`
- **Local mirror**: `/mnt/v/fuzzes/_farm/crashes/zenwebp/demux_container/x86_64/3374a77465dd395c`

Reproduce:
```
cd zenwebp
# copy the repro from the R2/local path above, then:
cargo +nightly fuzz run demux_container
```

Captured trace (tail)

```
elease/demux_container+0x130bb2) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#17 0x55a4d6799800 (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x134800) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#18 0x55a4d679d4b6 (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x1384b6) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#19 0x55a4d678dd1c (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x128d1c) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#20 0x55a4d678fb40 (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x12ab40) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#21 0x55a4d67a0684 (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x13b684) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#22 0x55a4d67a12c8 (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x13c2c8) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#23 0x55a4d679fd7d (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x13ad7d) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#24 0x55a4d67dbea8 (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x176ea8) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#25 0x55a4d67ae731 (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x149731) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#26 0x55a4d67b2fde (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x14dfde) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#27 0x55a4d67a12f6 (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x13c2f6) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)
#28 0x7139f9a2a1c9 (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 328820b908de8ea1ef79afa8995e302e819163d7)
#29 0x7139f9a2a28a (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 328820b908de8ea1ef79afa8995e302e819163d7)
#30 0x55a4d66cf444 (/root/work/zen/zenwebp/fuzz/target/x86_64-unknown-linux-gnu/release/demux_container+0x6a444) (BuildId: 11892ede965f61d75d116fed98a889c188cc4eab)

NOTE: libFuzzer has rudimentary signal handlers.
Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal
────────────────────────────────────────────────────────────────────────────────

Error: Fuzz target exited with exit status: 77

Stack backtrace:
0: anyhow::error::::msg
1: cargo_fuzz::project::FuzzProject::exec_fuzz
2: cargo_fuzz::main
3: std::sys::backtrace::__rust_begin_short_backtrace
4: std::rt::lang_start::{{closure}}
5: std::rt::lang_start_internal
6: main
7:
8: __libc_start_main
9: _start

```