signedness error caused broken decoding

Originally committed as revision 5277 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h263.c

@@ -553,7 +553,7 @@ void ff_clean_mpeg4_qscales(MpegEncContext *s){
 
 #endif //CONFIG_ENCODERS
 
-#define tab_size (sizeof(s->direct_scale_mv[0])/sizeof(int16_t))
+#define tab_size ((signed)(sizeof(s->direct_scale_mv[0])/sizeof(int16_t)))
 #define tab_bias (tab_size/2)
 
 static void ff_mpeg4_init_direct_mv(MpegEncContext *s){