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>
(cherry picked from commit 7ba0cedbfeff5671b264d1d7e90777057b5714c6)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit f06e39fe6b272a11782c023c31eec43bfce3138d)
pcx: Check the packet size before assuming it fits a palette

This fixes reads out of bounds.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit d1d99e3befea5d411ac3aae72dbdecce94f8b547)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

Conflicts:
	libavcodec/pcx.c
(cherry picked from commit 7e350b7ddd19af856b55634233d609e29baab646)