rtpjitterbuffer: Avoid generation of invalid timestamps

When updating timestamps and timer timeouts with a new offset, make sure that
the resulting value is valid (and not a negative (signed) value which ends up in
a massive (unsigned) value).

Fixes #571

Part-of: <gstreamer/gst-plugins-good!963>
31 jobs for !963 with backport-mr-960-into-1.18-jitter-signed in 25 minutes and 15 seconds (queued for 5 seconds)
merge request