Ensure top_active_prio_level_ is set to -1 in MaybeUpdateTopPrioLevel if last packet is purged. Bug: webrtc:15740 Change-Id: I81df9ee084de89f79b8ab79db8ce52fe1e20738a Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/333883 Reviewed-by: Erik Språng <sprang@webrtc.org> Commit-Queue: Per Kjellander <perkj@webrtc.org> Cr-Commit-Position: refs/heads/main@{#41498}