ff_gen_search: fix infinite loop

Fixes Ticket2639
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -1778,7 +1778,7 @@ int64_t ff_gen_search(AVFormatContext *s, int stream_index, int64_t target_ts,
             pos_max = FFMAX(0, pos_max - step);
             ts_max = ff_read_timestamp(s, stream_index, &pos_max, limit, read_timestamp);
             step += step;
-        }while(ts_max == AV_NOPTS_VALUE && pos_max > 0);
+        }while(ts_max == AV_NOPTS_VALUE && 2*limit > step);
         if (ts_max == AV_NOPTS_VALUE)
             return -1;