Fix null pointer dereference in ipc_boostrap

This was being hit when a sync message is dequeued and handled, and the
receiver endpoint immediately destroys itself.

BUG=714192

Review-Url: https://codereview.chromium.org/2834493008
Cr-Commit-Position: refs/heads/master@{#466383}
1 file changed