fix(xmpp): guard NULL domain in autoping disco warning gate

g_ascii_strcasecmp() is not NULL-safe (unlike the g_strcmp0 it replaced), and connection_get_domain() can be NULL (init NULL, FREE_SET_NULL on teardown). A disco#info response racing a disconnect would dereference NULL. Capture the domain and skip the check when it is NULL.

src/xmpp/iq.c

@@ -2432,7 +2432,8 @@ _disco_info_response_id_handler_onconnect(xmpp_stanza_t* const stanza, void* con
         }
 
         // Prevent repetitions by avoiding checks of disco items (from connection_set_disco_items)
-        if (from && g_ascii_strcasecmp(from, connection_get_domain()) == 0) {
+        const char* domain = connection_get_domain();
+        if (from && domain && g_ascii_strcasecmp(from, domain) == 0) {
             _disco_autoping_warning_message(features);
         }
     }