BUG/MINOR: lua: Fix a possible null pointer deref on lua ctx

This bug was introduced by the commit 8f587ea3 ("MEDIUM: lua: Set the analyse
expiration date with smaller wake_time only"). At the end of hlua_action(), the
lua context may be null if the alloc failed.

No backport needed, this is 2.3-dev.

src/hlua.c

@@ -6706,7 +6706,7 @@ static enum act_return hlua_action(struct act_rule *rule, struct proxy *px,
 	}
 
  end:
-	if (act_ret != ACT_RET_YIELD)
+	if (act_ret != ACT_RET_YIELD && s->hlua)
 		s->hlua->wake_time = TICK_ETERNITY;
 	return act_ret;
 }