Adds a nullptr check to prevent a rare crash when starting or stopping an RtcEventLog.

BUG=webrtc:4741,chromium:581788

Review URL: https://codereview.webrtc.org/1666843003

Cr-Original-Commit-Position: refs/heads/master@{#11490}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 20834ca806595a4f46c838f1e69c32d0c836fb12
1 file changed