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From: Anthony Hurtado <[redacted since hn has no scrape protection]>

vpk_read_packet() divides vpk->last_block_size and (par->block_align - vpk->last_block_size) by par->ch_layout.nb_channels without checking for zero.

While vpk_read_header() validates nb_channels > 0, the codec parameters may become zero through format probing misidentification (VPK probe score is 2/3 of AVPROBE_SCORE_MAX) or codec parameter reset, causing SIGFPE.

Fix by:

- Checking nb_channels != 0 before division in vpk_read_packet

- Returning EOF for empty last blocks (last_block_size == 0)

- Validating block_count > 0 in vpk_read_header

- Validating last_block_size <= block_align in vpk_read_header

Found by fuzzing with libFuzzer + AddressSanitizer. Reproduces with 10 distinct inputs.

[patch redacted for brevity]