Fix build. (dalvik-dev's GCC is smarter.)

Change-Id: I4b5cbe56878af2e852f9d84ddcf3e296e4814ead
diff --git a/src/mutex_test.cc b/src/mutex_test.cc
index fb14b9f..63aa14b 100644
--- a/src/mutex_test.cc
+++ b/src/mutex_test.cc
@@ -45,10 +45,13 @@
 TEST(Mutex, TryLockUnlock) {
   Mutex mu("test mutex");
   MutexTester::AssertDepth(mu, 0U);
-  ASSERT_TRUE(mu.TryLock());
-  MutexTester::AssertDepth(mu, 1U);
-  mu.Unlock();
-  MutexTester::AssertDepth(mu, 0U);
+  bool locked = mu.TryLock();
+  ASSERT_TRUE(locked);
+  if (locked) { // Keep GCC happy.
+    MutexTester::AssertDepth(mu, 1U);
+    mu.Unlock();
+    MutexTester::AssertDepth(mu, 0U);
+  }
 }
 
 // GCC doesn't get recursive mutexes, so we have to turn off thread safety analysis.