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>

libavcodec/rpza.c

@@ -204,7 +204,7 @@ static void rpza_decode_stream(RpzaContext *s)
 
         /* Fill block with 16 colors */
         case 0x00:
-            if (s->size - stream_ptr < 16)
+            if (s->size - stream_ptr < 30)
                 return;
             block_ptr = row_ptr + pixel_ptr;
             for (pixel_y = 0; pixel_y < 4; pixel_y++) {