rpza: limit the number of blocks to the total remaining blocks in the frame

Fixes invalid writes.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org
(cherry picked from commit 77bb0004bbe18f1498cfecdc68db5f10808b6599)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>