MXS-1839 Do not leak memory from maxinfo "show sessions"

When there is nothing more to show, the resultset user data
must be freed.

server/core/session.c

@@ -780,6 +780,10 @@ sessionRowCallback(RESULTSET *set, void *data)
         row = cbdata->row;
         cbdata->row = NULL;
     }
+    else
+    {
+        MXS_FREE(cbdata);
+    }
 
     return row;
 }