pcx: Consume the whole packet if giving up due to missing palette

Previously, we returned 0, meaning successful decoding but 0
bytes consumed, leading to an infinite loop.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>