mpeg4: use a self-explanatory #define MAX_NVOP_SIZE

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h263dec.c

@@ -459,7 +459,7 @@ retry:
         }
     }
 
-    if (s->bitstream_buffer_size && (s->divx_packed || buf_size < 20)) // divx 5.01+/xvid frame reorder
+    if (s->bitstream_buffer_size && (s->divx_packed || buf_size <= MAX_NVOP_SIZE)) // divx 5.01+/xvid frame reorder
         ret = init_get_bits8(&s->gb, s->bitstream_buffer,
                              s->bitstream_buffer_size);
     else

libavcodec/mpeg4video.h

@@ -59,6 +59,9 @@
 #define VISUAL_OBJ_STARTCODE 0x1B5
 #define VOP_STARTCODE        0x1B6
 
+/* smaller packets likely don't contain a real frame */
+#define MAX_NVOP_SIZE 19
+
 typedef struct Mpeg4DecContext {
     MpegEncContext m;