Mark error handler as called for closed sessions

When a DCB error occurs, the handleError entry point of the routers is
called. The caller of this entry point expects that the error handler
marks the DCB as handled. The aforementioned behavior is wrong as the
error handler should not keep track of whether the handler was already
called.

server/modules/routing/readwritesplit/readwritesplit.c

@@ -4439,6 +4439,7 @@ static void handleError(ROUTER *instance, void *router_session,
     if (!rses_begin_locked_router_action(rses))
     {
         /** Session is already closed */
+        problem_dcb->dcb_errhandle_called = true;
         *succp = false;
         return;
     }