virtio-gpu: avoid possible NULL pointer dereference

If output is NULL it is not permissable to dereference it.
So we should leave the respective function in this case.

The inconsistency was indicated by cppcheck.
No actual NULL pointer dereference was observed.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Message-id: 1471813566-5324-1-git-send-email-xypron.glpk@gmx.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>