mpeg4videodec: reset time_base.num to 0 when detecting time_base.den=0

to avoid leaving time_base in a broken/inconsistent state
fix issue2471

Originally committed as revision 26207 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpeg4videodec.c

@@ -1575,6 +1575,7 @@ static int decode_vol_header(MpegEncContext *s, GetBitContext *gb){
     s->avctx->time_base.den = get_bits(gb, 16);
     if(!s->avctx->time_base.den){
         av_log(s->avctx, AV_LOG_ERROR, "time_base.den==0\n");
+        s->avctx->time_base.num = 0;
         return -1;
     }