Allow MemMap to support empty map requests

Discovered trying to use ZipEntry::ExtractToMemMap on an empty preloaded-classes.

Bug: 8659045
Change-Id: Icfbbe6b313e2d1ee708b44a8d8d354d6203fd669
diff --git a/runtime/mem_map_test.cc b/runtime/mem_map_test.cc
new file mode 100644
index 0000000..dade01b
--- /dev/null
+++ b/runtime/mem_map_test.cc
@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2013 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "mem_map.h"
+
+#include "UniquePtr.h"
+#include "gtest/gtest.h"
+
+namespace art {
+
+class MemMapTest : public testing::Test {};
+
+TEST_F(MemMapTest, MapAnonymousEmpty) {
+  UniquePtr<MemMap> map(MemMap::MapAnonymous("MapAnonymousEmpty",
+                                             NULL,
+                                             0,
+                                             PROT_READ));
+  ASSERT_TRUE(map.get() != NULL);
+}
+
+}  // namespace art