missing malloc fail check in read_chunk_callback

[void0red]

we perform a test on pngquant and find two bugs in read_chunk_callback.

pngquant/rwpng.c

Line 147 in 4a48cfc

struct rwpng_chunk *chunk = malloc(sizeof(struct rwpng_chunk));

pngquant/rwpng.c

Line 151 in 4a48cfc

chunk->data = in_chunk->size ? malloc(in_chunk->size) : NULL;

it may cause NPD later since malloc fails when oom.

[kornelski]

Thanks!