Avoid copying and strlen(3) in the FindClass path.

Change-Id: I789f3c883596d1852a2c1954ce7a207e6f937117
diff --git a/src/dex2oat.cc b/src/dex2oat.cc
index ab63a48..a63d4ab 100644
--- a/src/dex2oat.cc
+++ b/src/dex2oat.cc
@@ -99,7 +99,7 @@
         continue;
       }
       std::string descriptor(DotToDescriptor(dot.c_str()));
-      SirtRef<Class> klass(class_linker->FindSystemClass(descriptor));
+      SirtRef<Class> klass(class_linker->FindSystemClass(descriptor.c_str()));
       if (klass.get() == NULL) {
         LOG(WARNING) << "Failed to find class " << descriptor;
         Thread::Current()->ClearException();