Fix segfault when no sources can't be found.
Fix segfault when no sources can't be found.

vmolsa/webrtc-native,vmolsa/webrtc-native,vmolsa/webrtc-native,vmolsa/webrtc-native,sukinull/webrtc-native,sukinull/webrtc-native,sukinull/webrtc-native,sukinull/webrtc-native,sukinull/webrtc-native,vmolsa/webrtc-native