Don't use UTF16 length as length for MUTF8.

Bug 11367555.

Change-Id: Ia0b07072a1a49d435c3b71ed9a668b316b7ff5d8
diff --git a/runtime/reflection.cc b/runtime/reflection.cc
index 4ff7349..80e16aa 100644
--- a/runtime/reflection.cc
+++ b/runtime/reflection.cc
@@ -323,7 +323,7 @@
   }
 
   JValue boxed_value;
-  const StringPiece src_descriptor(ClassHelper(o->GetClass()).GetDescriptorAsStringPiece());
+  const StringPiece src_descriptor(ClassHelper(o->GetClass()).GetDescriptor());
   mirror::Class* src_class = NULL;
   ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
   mirror::ArtField* primitive_field = o->GetClass()->GetIFields()->Get(0);