Fix use of invalid pointer in MediaStreamAudioSource.
If the source is not initiated correctly it can happen that the |audio_capturer_| is not set.
MediaStreamAudioSource::DoStop will then crash.

BUG= 340492
R=xians@chromium.org

Review URL: https://codereview.chromium.org/153643003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248763 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248763