rtpbuffer: Initialize extended timestamp to the first wraparound period

rtpbuffer: Initialize extended timestamp to the first wraparound period

This allows correct handling of wrapping around backwards during the
first wraparound period and avoids the infamous "Cannot unwrap, any
wrapping took place yet" error message.

It allows makes sure that for actual timestamp jumps a valid value is
returned instead of 0, which then allows the caller to handle it
properly. Not having this can have the caller see the same timestamp (0)
for a very long time, which for example can cause rtpjitterbuffer to
output the same timestamp for a very long time.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1500

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3202>