ffplay: disallow seeking before the start of the file

In timestamp based seeking we update the external clock to the seek target,
therefore we should use sane timestamps even if libavformat could handle
seeking before the start of the file.
Signed-off-by: Marton Balint <cus@passwd.hu>

ffplay.c

@@ -2984,6 +2984,8 @@ static void event_loop(VideoState *cur_stream)
                     } else {
                         pos = get_master_clock(cur_stream);
                         pos += incr;
+                        if (cur_stream->ic->start_time != AV_NOPTS_VALUE && pos < cur_stream->ic->start_time / (double)AV_TIME_BASE)
+                            pos = cur_stream->ic->start_time / (double)AV_TIME_BASE;
                         stream_seek(cur_stream, (int64_t)(pos * AV_TIME_BASE), (int64_t)(incr * AV_TIME_BASE), 0);
                     }
                 break;