gnu: imagemagick: Update to 6.9.9-18.

Fixes <https://bugs.gnu.org/28692>.
Reported by Diego Nicola Barbato <dnbarbato@posteo.de>.

* gnu/packages/imagemagick.scm (imagemagick): Update to 6.9.9-18.

gnu/packages/imagemagick.scm

@@ -46,14 +46,14 @@ (define-public imagemagick
     ;; The 7 release series has an incompatible API, while the 6 series is still
     ;; maintained. Don't update to 7 until we've made sure that the ImageMagick
     ;; users are ready for the 7-series API.
-    (version "6.9.9-17")
+    (version "6.9.9-18")
     (source (origin
              (method url-fetch)
              (uri (string-append "mirror://imagemagick/ImageMagick-"
                                  version ".tar.xz"))
              (sha256
               (base32
-               "0y39jcd6358dph51ch5w43sqk9lv079jhgg3l7g5mks6m25f49gz"))))
+               "1d70dvrgcasa3cla4c1dag4mw7vq4jk1nzbd604fk20l0l6xa78j"))))
     (build-system gnu-build-system)
     (arguments
      `(#:configure-flags '("--with-frozenpaths" "--without-gcc-arch")