Avoid some crashes in the process scanner thread

get_window_process() can fail for many reasons, one of them being lack
of permission to inject into some processes.

Before this change, the process scanner thread could crash by simply
launching the Task Manager (taskmgr.exe), before we injected into
Spotify.

I also added a check for event.window_handle(), just to be safe.

burnt-sushi/src/spotify_process_scanner.rs

@@ -144,8 +144,12 @@ impl SpotifyProcessScanner {
         while let Some(event) = event_rx.recv().await {
             // scoped to make future Send
             let state = {
-                let window = event.window_handle().unwrap();
-                let process = get_window_process(window)?;
+                let Some(window) = event.window_handle() else {
+                    continue;
+                };
+                let Ok(process) = get_window_process(window) else {
+                    continue;
+                };
                 if !is_spotify_process(process.borrowed()) || !is_main_spotify_window(window) {
                     continue;
                 }