chan_sip: Fix reference leak in mwi_event_cb

Cleanup the peer reference when stasis_subscription_final_message is
true.  Also free peer_name even if peer exists, after reload a new
peer_name will be allocated.

ASTERISK-26193 #close

Change-Id: If7ecd52facdc5c227f701c760841e3f6ca53cc69

channels/chan_sip.c

@@ -17186,10 +17186,8 @@ static void mwi_event_cb(void *userdata, struct stasis_subscription *sub, struct
 	struct sip_peer *peer = sip_find_peer(peer_name, NULL, TRUE, FINDALLDEVICES, FALSE, 0);
 
 	if (stasis_subscription_final_message(sub, msg)) {
-		if (peer) {
-			/* configuration reloaded */
-			return;
-		}
+		/* peer can be non-NULL during reload. */
+		ao2_cleanup(peer);
 		ast_free(peer_name);
 		return;
 	}