With the current order of stop capture processing on Android,
OnMemoryBufferFrame and OnTextureFrame may be called halfway through
Stop(). They must therefore check for the case of a null capturer_.
There used to be such checks, but they were accidantally removed in
commit #12895, cl https://codereview.webrtc.org/1973873003.
BUG=webrtc:5966
R=perkj@webrtc.org, sakal@webrtc.org
Review URL: https://codereview.webrtc.org/2033943004 .
Cr-Commit-Position: refs/heads/master@{#13031}
47183ba60b