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 77bb0004bb)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/rpza.c

@@ -38,6 +38,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "libavutil/common.h"
 #include "libavutil/internal.h"
 #include "libavutil/intreadwrite.h"
 #include "avcodec.h"
@@ -126,6 +127,8 @@ static void rpza_decode_stream(RpzaContext *s)
             }
         }
 
+        n_blocks = FFMIN(n_blocks, total_blocks);
+
         switch (opcode & 0xe0) {
 
         /* Skip blocks */