splitmuxsink: Require buffers to have a valid PTS

If we receive a buffer without a valid PTS or DTS, running_time_dts will be NONE, but splitmux->max_in_running_time_dts might be valid from before. As a result, the running_time_dts < next_gop->start_time_pts check in handle_mq_input will not fire (because it also checks that running_time_dts != GST_CLOCK_STIME_NONE), the main context will go into check_completed_gop, making it return on splitmux->max_in_running_time_dts < next_gop->start_time_pts , and the next iteration will hit the g_assert (!ctx->is_reference);