[3.0] demux: wav: check the size of the fmt chunk is legit
WAVEFORMATEX.cbSize represents the size of data after the WAVEFORMATEX structure 1. It is coded on a uint16_t so the size of the WAVEFORMATEX plus its extra data can never exceed sizeof(WAVEFORMATEX) + UINT16_MAX.
Fixes #29004 (closed)
(cherry picked from commit d5ab4348)
Backport of !7549 (merged) (extracted from !7558)