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/build/Android.gtest.mk b/build/Android.gtest.mk
index ad33365..6a73fc0 100644
--- a/build/Android.gtest.mk
+++ b/build/Android.gtest.mk
@@ -39,6 +39,7 @@
 	runtime/indirect_reference_table_test.cc \
 	runtime/intern_table_test.cc \
 	runtime/jni_internal_test.cc \
+	runtime/mem_map_test.cc \
 	runtime/mirror/dex_cache_test.cc \
 	runtime/mirror/object_test.cc \
 	runtime/oat/utils/arm/managed_register_arm_test.cc \