chan_iax2: On reload make sure to check for existing MWI subscription

On every reload of chan_iax2 module, MWI subscription was added, which
results in additional taskprocessors being accumulated over time.

This commit fixes it by making sure we check for existing subscription
first.

This was verified with 'core show taskprocessors' CLI command.

ASTERISK-27122 #close

Change-Id: Ie2ef528fd5ca01b933eeb88188cc10967899cfb9

channels/chan_iax2.c

@@ -13084,7 +13084,7 @@ static struct iax2_peer *build_peer(const char *name, struct ast_variable *v, st
 		ast_free_acl_list(oldacl);
 	}
 
-	if (!ast_strlen_zero(peer->mailbox)) {
+	if (!ast_strlen_zero(peer->mailbox) && !peer->mwi_event_sub) {
 		struct stasis_topic *mailbox_specific_topic;
 
 		mailbox_specific_topic = ast_mwi_topic(peer->mailbox);