Bug 963242 - Add null checks in AndroidBridge::JavaThreadDetachFunc() r=kats

widget/android/AndroidBridge.cpp

@@ -1566,8 +1566,16 @@ static void
 JavaThreadDetachFunc(void *arg)
 {
     JNIEnv *env = (JNIEnv*) arg;
+    MOZ_ASSERT(env, "No JNIEnv on Gecko thread");
+    if (!env) {
+        return;
+    }
     JavaVM *vm = nullptr;
     env->GetJavaVM(&vm);
+    MOZ_ASSERT(vm, "No JavaVM on Gecko thread");
+    if (!vm) {
+        return;
+    }
     vm->DetachCurrentThread();
 }