nmtui/wireguard: fix peer leak when adding new peer

nm_wireguard_peer_new() returns a new peer with refcount 1.
nmt_wireguard_peer_editor_new() takes its own copy via
g_value_dup_boxed(), so the original peer was never unreffed.

Unref the peer after passing it to the editor.

Found by Coverity (CID: RESOURCE_LEAK).

Fixes: b0f5b1d97a ('tui: add WireGuard support to nmtui')
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
(cherry picked from commit 00f9a3d862)

src/nmtui/nmt-wireguard-peer-list.c

@@ -170,6 +170,7 @@ nmt_wireguard_peer_list_add_peer(NmtWireguardPeerList *list)
     NmtNewtForm                 *editor;
 
     editor = nmt_wireguard_peer_editor_new(priv->setting, peer);
+    nm_wireguard_peer_unref(peer);
 
     if (!editor)
         return;