[mojo-edk] Fix bootstrap NodeChannel Start/ShutDown race
[mojo-edk] Fix bootstrap NodeChannel Start/ShutDown race

If the parent bootstrap signals an error very early in
NodeController startup, it can be ShutDown() before
Start() is invoked on the IO thread via
ConnectoToParentOnIOThread.

This changes Start() to gracefully handle a null
|channel_|.

BUG=610337
R=amistry@chromium.org

Review-Url: https://codereview.chromium.org/1985993002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394055}

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