Fix JS error on app quit (#377)

This was a rare race condition during app shutdown where a 'wt-'
message would be sent from the hidden webtorrent window to the main
window after the main window was already closed.

Fixes #373

main/windows.js

@@ -72,6 +72,10 @@ function createWebTorrentHiddenWindow () {
       win.hide()
     }
   })
+
+  win.once('closed', function () {
+    windows.webtorrent = null
+  })
 }
 
 function createMainWindow () {