mojo: Fix mojo test on Art runtime.

mojo/android/javatests/validation_test_util.cc

Index: mojo/android/javatests/validation_test_util.cc
diff --git a/mojo/android/javatests/validation_test_util.cc b/mojo/android/javatests/validation_test_util.cc
index 31aa3699b1f3cb7f79fe79e5fe63f4c8453b1616..2289e025a838b3bd70b954763d2a908bce0a1e1a 100644
--- a/mojo/android/javatests/validation_test_util.cc
+++ b/mojo/android/javatests/validation_test_util.cc
@@ -31,9 +31,13 @@ jobject ParseData(JNIEnv* env, jclass jcaller, jstring data_as_string) {
     return Java_ValidationTestUtil_buildData(
                env, NULL, 0, j_error_message.obj()).Release();
   }
-
+  void* data_ptr = &data[0];
+  if (!data_ptr) {
+    DCHECK(!data.size());
+    data_ptr = &data;
+  }
   jobject byte_buffer =
-      env->NewDirectByteBuffer(&data[0], data.size());
+      env->NewDirectByteBuffer(data_ptr, data.size());
   return Java_ValidationTestUtil_buildData(env, byte_buffer, num_handles, NULL)
       .Release();
 }