basesink: fix use of uninitialized variable

basesink: fix use of uninitialized variable

Running suite(s): GstBaseSink
==22023== Conditional jump or move depends on uninitialised value(s)
==22023==    at 0x505FFCE: gst_base_sink_get_sync_times (gstbasesink.c:1936)
==22023==    by 0x5068C80: gst_base_sink_do_sync (gstbasesink.c:2379)
==22023==    by 0x506BCD2: gst_base_sink_default_wait_event (gstbasesink.c:2903)
==22023==    by 0x50633A4: gst_base_sink_default_event (gstbasesink.c:2918)
==22023==    by 0x6F5C216: gst_fake_sink_event (gstfakesink.c:383)
==22023==    by 0x505F164: gst_base_sink_event (gstbasesink.c:3108)
==22023==    by 0x52FA090: gst_pad_send_event_unchecked (gstpad.c:4822)
==22023==    by 0x5303756: gst_pad_send_event (gstpad.c:4984)
==22023==    by 0x40165B: basesink_test_gap (basesink.c:148)