mojo: Fix some crashing app-tests.
mojo: Fix some crashing app-tests.

The MessageLoop may have been destroyed by the time the termination-closure
runs. So check for the MessageLoop's existence before attempting to terminate
it.

BUG=496935
TBR=jam@ to green the bots

Review URL: https://codereview.chromium.org/1223133008

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#338958}

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