Remove an NPE in tearDown

This is masking an exception in setUp() and makes debugging difficult

Test: treehugger
Change-Id: I8b5b80c4171d45076dd02bab5ff4b6dc81d98086

tests/cts/hostside/app/src/com/android/cts/net/hostside/AbstractRestrictBackgroundNetworkTestCase.java

@@ -198,7 +198,8 @@ public abstract class AbstractRestrictBackgroundNetworkTestCase {
     protected void tearDown() throws Exception {
         executeShellCommand("cmd netpolicy stop-watching");
         mServiceClient.unbind();
-        if (mLock.isHeld()) mLock.release();
+        final PowerManager.WakeLock lock = mLock;
+        if (null != lock && lock.isHeld()) lock.release();
     }
 
     protected int getUid(String packageName) throws Exception {