Fix NPE in NsdServiceTest

Continuous test dashboards report that quit() can crash sometimes
due to mThread being null.

This patch adds a null guard in tearDown().

Bug: 32561414
Test: runtest frameworks-net
Change-Id: If66fb47e31e77d25b4741a786f12eb78f0b9102e

tests/net/java/com/android/server/NsdServiceTest.java

@@ -77,7 +77,10 @@ public class NsdServiceTest {
 
     @After
     public void tearDown() throws Exception {
+        if (mThread != null) {
             mThread.quit();
+            mThread = null;
+        }
     }
 
     @Test