update dialog: Avoid crashing when the dialog is quickly closed

Be more careful to avoid dereferencing priv variables in an async
callback handler, which can be called when the dialog is in dispose and
the priv variables have all already been cleared to NULL.

Note that this needs fixing in many more places, but this particular
crash is a top F30 crasher.

https://bugzilla.redhat.com/show_bug.cgi?id=1634075
1 job for wip/kalev/crash-when-quickly-closing-update-dialog in 18 minutes (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #289617
build-gnome-software

00:18:00