Dont try generic seek if seek request before first index entry and backward.

Fixes issue1275

Originally committed as revision 21633 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -1548,6 +1548,9 @@ static int av_seek_frame_generic(AVFormatContext *s,
 
     index = av_index_search_timestamp(st, timestamp, flags);
 
+    if(index < 0 && st->nb_index_entries && timestamp < st->index_entries[0].timestamp)
+        return -1;
+
     if(index < 0 || index==st->nb_index_entries-1){
         int i;
         AVPacket pkt;