Fixes potential crash in SendSideCongestionController when route is
changed before network is available.
Bug: webrtc:8415
Change-Id: I781f0e342e5bb42fedbf96c9c5c6d2c199ab3192
Reviewed-on: https://webrtc-review.googlesource.com/69801
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22851}