rpza: Fix a buffer size check

We read 2 bytes for 15 out of 16 pixels, therefore we need to
have at least 30 bytes, not 16.

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