Avoid crash in ViEEncoder::DeRegisterExternalEncoder().
Avoid crash in ViEEncoder::DeRegisterExternalEncoder().

BUG=chromium:348222
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/9519004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 64e0405552932e492fbb0de181e3fa06fb3c45ce

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc