Xwayland: fix possible use-after-free on unmap

The view.unmap() call may synchronously destroy the view, which makes
the the code removing listeners which is currently run after
view.unmap() access free'd memory.

To fix this, simply change the order of the calls to match that of
XdgToplevel.handleUnmap().

river/XwaylandView.zig

@@ -214,12 +214,12 @@ fn handleMap(listener: *wl.Listener(*wlr.XwaylandSurface), xwayland_surface: *wl
 fn handleUnmap(listener: *wl.Listener(*wlr.XwaylandSurface), xwayland_surface: *wlr.XwaylandSurface) void {
     const self = @fieldParentPtr(Self, "unmap", listener);
 
-    self.view.unmap();
-
     // Remove listeners that are only active while mapped
     self.commit.link.remove();
     self.set_title.link.remove();
     self.set_class.link.remove();
+
+    self.view.unmap();
 }
 
 fn handleRequestConfigure(