Fix an NPE in WatchdogEventLogger#stop

See b/152550404#comment15.

https://cs.corp.google.com/android/frameworks/base/tests/RollbackTest/StagedRollbackTest/src/com/android/tests/rollback/host/StagedRollbackTest.java?rcl=0b9230489ead3a2ad328d46a47194cfa9dffd25c&l=86
My theory is that somehow #setUp throws before calling mLogger.start()
and then mLogger.stop() runs into an NPE.

Let's add a null-check in the hope that the actual root cause will stand
out.

Bug: 152550404
Test: atest StagedRollbackTest
Change-Id: Ifd323e6a8707a2de19d03ecbe536feacdbb4e9c6