dont assert(0) due to buggy rm demuxer

Originally committed as revision 8790 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -1217,6 +1217,10 @@ int64_t av_gen_search(AVFormatContext *s, int stream_index, int64_t target_ts, i
 #ifdef DEBUG_SEEK
 av_log(s, AV_LOG_DEBUG, "%"PRId64" %"PRId64" %"PRId64" / %"PRId64" %"PRId64" %"PRId64" target:%"PRId64" limit:%"PRId64" start:%"PRId64" noc:%d\n", pos_min, pos, pos_max, ts_min, ts, ts_max, target_ts, pos_limit, start_pos, no_change);
 #endif
+        if(ts == AV_NOPTS_VALUE){
+            av_log(s, AV_LOG_ERROR, "read_timestamp() failed in the middle\n");
+            return -1;
+        }
         assert(ts != AV_NOPTS_VALUE);
         if (target_ts <= ts) {
             pos_limit = start_pos - 1;