Fix TOCTOU race in RtpVideoSender::OnVideoLayersAllocationUpdated

RtpVideoSender::OnVideoLayersAllocationUpdated runs on the encoder thread,
checks if it's active, and then posts a task to the transport queue to
update module status.

If SetSending(false) is called on the transport queue before the posted
task runs, the task would previously re-register modules in PacketRouter
even though active_ was false. This lead to dangling pointers in
PacketRouter when the RtpVideoSender was destroyed.

This CL adds an IsActive() check inside the posted task to ensure we don't
re-register modules if the sender has been deactivated.

Bug: chromium:500767595
Test: RtpVideoSenderTest.PostTaskRaceDoesNotLeadToDanglingPointer
Change-Id: I18827c34bece7f5ebc41d159d9e19e356afbc404
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/462880
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47384}