vmsvga: correct bitmap and pixmap size checks

When processing svga command DEFINE_CURSOR in vmsvga_fifo_run,
the computed BITMAP and PIXMAP size are checked against the
'cursor.mask[]' and 'cursor.image[]' array sizes in bytes.
Correct these checks to avoid OOB memory access.

Reported-by: Qinghao Tang <luodalongde@gmail.com>
Reported-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-id: 1473338754-15430-1-git-send-email-ppandit@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
