BUG/MINOR: spoe: do not assume agent->rt is valid on exit

As reported by Christopher, we may call spoe_release_agent() when leaving
after an allocation failure or a config parse error. We must not assume
agent->rt is valid there as the allocation could have failed.

This should be backported to 1.9 and 1.8.

src/flt_spoe.c

@@ -184,8 +184,10 @@ spoe_release_agent(struct spoe_agent *agent)
 		LIST_DEL(&grp->list);
 		spoe_release_group(grp);
 	}
+	if (agent->rt) {
 		for (i = 0; i < global.nbthread; ++i)
 			HA_SPIN_DESTROY(&agent->rt[i].lock);
+	}
 	free(agent->rt);
 	free(agent);
 }