Re-add missing return in RegisterExternalDecoder.
Re-add missing return in RegisterExternalDecoder.

Breaks waterfall due to possible null-pointer dereferences.

BUG=webrtc:5249
TBR=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1483623002 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10824}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 395c7c65194988f1102f461ed9ff0cf932f72748

aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk