The size==1 (64-bit "largesize") branch of all three MP4 box walkers
(findMP4Box, readMp4DurationBytes, readMp4Duration) set boxEnd to the raw
largesize instead of offset+largesize — even though the 32-bit branch right
below correctly uses offset+size. Two consequences:
- Correctness: for any MP4 that carries a 64-bit box size at a non-zero
offset, the box walk is computed from the wrong end, so the moov/mvhd
lookup is truncated and the media duration is silently lost.
- Robustness/security (CWE-190): the unguarded uint64->int(64) conversion of
a largesize with the high bit set yields a negative boxEnd. The in-memory
walkers then assign it to offset and feed it back as a slice index
(data[offset:]), panicking with "slice bounds out of range" and crashing
the CLI on a crafted or corrupt MP4. This is reachable via URL-sourced IM
media, whose bytes the caller does not control.
Fix: compute boxEnd as offset+largesize (matching the 32-bit branch) and
reject largesize values smaller than the 16-byte header or larger than the
remaining input. Malformed media now honours the parsers' best-effort
contract by returning 0/-1 instead of panicking, and the bounds guarantee
the conversion can no longer overflow.
Add regression tests covering both the overflow (must not panic) and a
64-bit box at a non-zero offset (must walk correctly).
