shill: Fix a crash when terminating daemon in presence of cellular device

This CL fixes a crash caused by destroying a Cellular object in midst of
executing the completion callback of its termination action.

BUG=chromium:383767
TEST=Tested the following:
1. `FEATURES=test emerge-$BOARD platform2`
2. On a DUT with a cellular device in the enabled state, verify that
   `restart shill` will cause shill to execute the termination action
   without crashing.

Change-Id: Ia521b00688816961e25f74dd87d81176220e7ea9
Reviewed-on: https://chromium-review.googlesource.com/203561
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>