Fix segfault in multithreaded context

Dropping libwayland-client's queue in multithreaded context is safe if
and only if there're no alive proxies on it, otherwise it could result
in use-after-free. For more see [1] and [2].

[1] - https://gitlab.freedesktop.org/wayland/wayland/-/issues/13
[2] - https://github.com/Smithay/wayland-rs/issues/359

examples/clipboard.rs

@@ -223,7 +223,7 @@ fn process_keyboard_event(event: KeyboardEvent, dispatch_data: &mut DispatchData
             let contents = dispatch_data
                 .clipboard
                 .load()
-                .unwrap_or_else(|_| String::from("Failed to load primary selection"));
+                .unwrap_or_else(|_| String::from("Failed to load selection"));
             println!("Paste: {}", contents);
         }
         // Copy primary.