fix return type in Statistics::lookup method

Change-Id: Ide0b7382e607c97411868035c52c908895fbdcb0
Reviewed-on: https://gerrit.dechocorp.com/29163
Reviewed-by: Jeremy Stanley <jeremy@mozy.com>
Reviewed-by: Kevin Cai <kcai@vmware.com>

mordor/statistics.h

@@ -312,7 +312,7 @@ class Statistics
         } else {
             T *stat = dynamic_cast<T *>(it->second.second.get());
             MORDOR_ASSERT(stat);
-            return *stat;
+            return stat;
         }
     }