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
diff --git a/src/com/android/bluetooth/pan/PanService.java b/src/com/android/bluetooth/pan/PanService.java
index 22f4d83..36add63 100755
--- a/src/com/android/bluetooth/pan/PanService.java
+++ b/src/com/android/bluetooth/pan/PanService.java
@@ -441,7 +441,7 @@
                     ifaceAddr = null;
                 }
             }
-        } else {
+        } else if (mTetherAc != null) {
             // PANU Role = reverse Tether
             Log.d(TAG, "handlePanDeviceStateChange LOCAL_PANU_ROLE:REMOTE_NAP_ROLE state = " +
                     state + ", prevState = " + prevState);