rtponviftimestamp: Fix drop-out-of-segment=false mode

rtponviftimestamp: Fix drop-out-of-segment=false mode

Fixing unexpected buffer dropping and flow error in case that:
* use-reference-timestamps=false
* drop-out-of-segment=false
* Calculated utc offset is not valid because buffer is out-of-segment

The above case should be considered as a valid data flow without returning
errors.

Fixing regression introduced by
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1683

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5788>