imagemagick: security bump to 7.0.2-9

Fixes a number of buffer overflows / use-after-free issues:
http://git.imagemagick.org/repos/ImageMagick/blob/master/ChangeLog

  * Prevent buffer overflow in BMP & SGI coders (bug report from
    pwchen&rayzhong of tencent).
  * Prevent buffer overflow and other problems in SIXEL, PDB, MAP, TIFF and
    CALS coders (bug report from Donghai Zhu).
  * Prevent buffer overflow (bug report from Max Thrane).
  * Prevent memory use after free (reference
    https://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=30245

).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>