gnu: imagemagick: Update to 6.9.12-2 [security fixes].

* gnu/packages/imagemagick.scm (imagemagick/fixed): New variable.
(imagemagick)[replacement]: Graft.

gnu/packages/imagemagick.scm

@@ -58,6 +58,7 @@
              (sha256
               (base32
                "0m8nkmywkqwyrr01q7aiakj6mi4rb2psjgzv8n0x82x3s1rpfyql"))))
+    (replacement imagemagick/fixed)
     (build-system gnu-build-system)
     (arguments
      `(#:configure-flags '("--with-frozenpaths" "--without-gcc-arch"
@@ -126,6 +127,18 @@ transform images, adjust image colors, apply various special effects, or draw
 text, lines, polygons, ellipses and Bézier curves.")
     (license (license:fsf-free "http://www.imagemagick.org/script/license.php"))))
 
+(define-public imagemagick/fixed
+  (package
+    (inherit imagemagick)
+    (version "6.9.12-2")
+    (source (origin
+              (method url-fetch)
+              (uri (string-append "mirror://imagemagick/ImageMagick-"
+                                  version ".tar.xz"))
+              (sha256
+               (base32
+                "17da5zihz58qm41y61sbvw626m5xfwr2nzszlikrvxyq1j1q7asa"))))))
+
 (define-public perl-image-magick
   (package
     (name "perl-image-magick")