Keep update available flag set until restart

lib/browser/api/auto-updater/auto-updater-win.js

@@ -34,7 +34,6 @@ class AutoUpdater extends EventEmitter {
         return this.emitError(error)
       }
       if (update == null) {
-        this.updateAvailable = false
         return this.emit('update-not-available')
       }
       this.updateAvailable = true