Fix DesktopAndCursorComposer not to crash

DesktopAndCursorComposer was crashing when screen/window
capturer returns a NULL frame due to an error.

BUG=crbug.com/344093
R=jiayl@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/8769004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5573 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed