THRIFT-545.  Fix mem leak in generated 'description' method

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@796391 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/compiler/cpp/src/generate/t_cocoa_generator.cc b/compiler/cpp/src/generate/t_cocoa_generator.cc
index 835c84d..fab0c54 100644
--- a/compiler/cpp/src/generate/t_cocoa_generator.cc
+++ b/compiler/cpp/src/generate/t_cocoa_generator.cc
@@ -966,7 +966,7 @@
   }
   out <<
     indent() << "[ms appendString: @\")\"];" << endl <<
-    indent() << "return [ms copy];" << endl;
+    indent() << "return [NSString stringWithString: ms];" << endl;
 
   indent_down();
   indent(out) << "}" << endl <<