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When a Read() call finished asynchronously, it was clearing its buf_
pointer after calling into its consumer. If its consumer called Read()
again, that could have resulted in populating buf_ again, which would
then incorrectly be cleared by the higher level call.

Also, if Init() interrupted a previous call to Init(), the callback to
the first Init() call was not being cancelled.

Bug: 794330
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I71faee5bcde302acd1962e55b4f2bb75d8c31c8d
Reviewed-on: https://chromium-review.googlesource.com/825843
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Randy Smith <rdsmith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524448}