Fix potential NPE in PbapClientStateMachine

If the PbapClientStateMachine is destroyed right after
it is created, the exit() method of the init state could be
called without first calling the enter().

Bug: 290976844
Test: atest PbapClientServiceTest 100 times
Change-Id: Ia034526009c69d9194eff5e26635b011c64607d1