bugfix: use THD::main_mem_root for kill error message

cannot use the current THD::mem_root, because it can be temporarily
reassigned to something with a very different life time
(e.g. to TABLE::mem_root or range optimizer mem_root).

sql/sql_class.h

@@ -3729,7 +3729,8 @@ class THD :public Statement,
           The worst things that can happen is that we get
           a suboptimal error message.
         */
-        if ((killed_err= (err_info*) alloc(sizeof(*killed_err))))
+        killed_err= (err_info*) alloc_root(&main_mem_root, sizeof(*killed_err));
+        if (killed_err)
         {
           killed_err->no= killed_errno_arg;
           ::strmake((char*) killed_err->msg, killed_err_msg_arg,