mpeg4videodec: dont treat empty frames as errors for QMP4, it seems thats normal.

Fixes remaining complaints in Ticket560 possibly.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg4videodec.c

@@ -2139,7 +2139,7 @@ int ff_mpeg4_decode_picture_header(MpegEncContext * s, GetBitContext *gb)
     startcode = 0xff;
     for(;;) {
         if(get_bits_count(gb) >= gb->size_in_bits){
-            if(gb->size_in_bits==8 && (s->divx_version>=0 || s->xvid_build>=0)){
+            if(gb->size_in_bits==8 && (s->divx_version>=0 || s->xvid_build>=0) || s->codec_tag == AV_RL32("QMP4")){
                 av_log(s->avctx, AV_LOG_WARNING, "frame skip %d\n", gb->size_in_bits);
                 return FRAME_SKIPPED; //divx bug
             }else