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}