avoid null pointer in handlePanDeviceStateChange of PanService

see the following null pointer exception
04-30 15:15:21.689 10890 10890 E AndroidRuntime: Caused by: java.lang.NullPointerException
04-30 15:15:21.689 10890 10890 E AndroidRuntime: 	at com.android.bluetooth.pan.PanService.handlePanDeviceStateChange(PanService.java:457)
which cause crash when disable BT
If the stop is called before the cleanup in PanService, Nullpointer may happen.

issue 8769031

Change-Id: Ia3fb1625c945c508771a697234e6ee18a120d72b