identity: Fix crash when receiving a gap event outside the current segment

We were checking if the start time of the gap event was
GST_CLOCK_TIME_NONE, which is superfluous because that cannot happen,
and then not checking if it was NONE after gst_segment_to_running_time,
which caused a crash if an identity received a gap event fully or
partially outside the current segment.

This patch was done in cooperation with:
Jan Alexander Steffens (heftig) <jan.steffens@ltnglobal.com>

Part-of: <gstreamer/gstreamer!1269>
26 jobs for !1269 with identity_gasp_outside_segment in 108 minutes and 5 seconds (queued for 8 seconds)
merge request