Fix one NPE in tethering setting

Once we not declare BT feature, go Setting->more->tethering
will trigger this NPE.

Change-Id: I4736e7bade261475acda74e26250a81452018ffa
Signed-off-by: Jianzheng Zhou <jianzheng.zhou@freescale.com>

src/com/android/settings/TetherSettings.java

@@ -385,6 +385,8 @@ public class TetherSettings extends SettingsPreferenceFragment
         }
 
         BluetoothAdapter adapter = BluetoothAdapter.getDefaultAdapter();
+        if (adapter == null)
+            return;
         int btState = adapter.getState();
         if (btState == BluetoothAdapter.STATE_TURNING_OFF) {
             mBluetoothTether.setEnabled(false);