'address_space_get_flatview' gets a reference to a FlatView.
If the flatview lookup fails, the code returns without
"unreferencing" the view.

Cc: qemu-stable@nongnu.org

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>