Protect BackStackRecord from accessing null mHost.

Bug 33177567

The fragment manager is being accessed after it has been torn
down. This protects BackStackRecord from accessing the null
mHost in that case.

Test: cts-tradefed run singleCommand cts -d --skip-preconditions
--skip-connectivity-check -m CtsFragmentTestCases

Change-Id: Id927eb1bc78e3750b01f685523418bde5a9cf8a0
1 file changed