Merge remote-tracking branch 'remotes/kraxel/tags/vga-20180125-pull-request' into staging

vga: fix for CVE-2018-5683

# gpg: Signature made Thu 25 Jan 2018 09:33:23 GMT
# gpg:                using RSA key 0x4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/vga-20180125-pull-request:
  vga: check the validation of memory addr when draw text

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>