Fix timestamp association for mpeg2 field pictures.

Fixes /MPlayer/incoming/codec_copy_fails_vob_to_mpeg-ts/codec_copy_fails_vob_to_mpeg-ts.vob

Originally committed as revision 21949 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpeg12.c

@@ -2204,7 +2204,7 @@ int ff_mpeg1_find_frame_end(ParseContext *pc, const uint8_t *buf, int buf_size,
                     return i-3;
                 }
             }
-            if(s && state == PICTURE_START_CODE){
+            if(pc->frame_start_found == 0 && s && state == PICTURE_START_CODE){
                 ff_fetch_timestamp(s, i-3, 1);
             }
         }