Don't default-construct std::strings and then assign them.

Change-Id: I8c994d1e6a8d2ebe52eaa4f0132e0deb2ecfa5f3
diff --git a/src/reflection.cc b/src/reflection.cc
index a6ae096..46b2ff3 100644
--- a/src/reflection.cc
+++ b/src/reflection.cc
@@ -305,7 +305,7 @@
   }
 
   JValue boxed_value = { 0 };
-  std::string src_descriptor = ClassHelper(o->GetClass()).GetDescriptor();
+  std::string src_descriptor(ClassHelper(o->GetClass()).GetDescriptor());
   Class* src_class = NULL;
   ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
   Field* primitive_field = o->GetClass()->GetIFields()->Get(0);