We land here in case not everything we've asked for could be mapped.
So unmap only the bytes which have actually been mapped.

Also we didn't access anything, so acces_len can be 0.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-id: 20190628072357.31782-1-kraxel@redhat.com