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 #1500 (closed)