package/gd: fix CVE-2018-14553 and CVE-2019-6977

- Fix CVE-2018-14553 : gdImageClone in gd.c in libgd 2.1.0-rc2 through
  2.2.5 has a NULL pointer dereference allowing attackers to crash an
  application via a specific function call sequence.

- Fix CVE-2019-6977: gdImageColorMatch in gd_color_match.c in the GD
  Graphics Library (aka LibGD) 2.2.5, as used in the imagecolormatch
  function in PHP before 5.6.40, 7.x before 7.1.26, 7.2.x before 7.2.14,
  and 7.3.x before 7.3.1, has a heap-based buffer overflow. This can be
  exploited by an attacker who is able to trigger imagecolormatch calls
  with crafted image data.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>