If stopCapture is called shortly after startCapture, and the first startCaptureOnCameraThread failed, but still hasn't retried 3 times, stopCaptureOnCameraThread will be called in a state where the camera is not initialized. This CL adds null checks in stopCaptureOnCameraThread to avoid crashes.
BUG=b/27939867
Review URL: https://codereview.webrtc.org/1854103002
Cr-Commit-Position: refs/heads/master@{#12234}