magick-load: use more robust g_spawn_async() instead of system()

This fixes issue #298 by avoiding the shell parsing being invoked at
all, this less brittle than any forms of escaping characters, while
retaining the ability to address all existing files.

operations/common/magick-load.c

@@ -41,20 +41,23 @@ load_cache (GeglProperties *op_magick_load)
   if (!op_magick_load->user_data)
     {
       gchar    *filename;
-      gchar    *cmd;
       GeglNode *graph, *sink, *loader;
       GeglBuffer *newbuf = NULL;
 
       /* ImageMagick backed fallback FIXME: make this robust.
        * maybe use pipes in a manner similar to the raw loader,
        * or at least use a properly unique filename  */
+      char     *argv[4]  = {"convert", NULL, NULL, NULL};
 
       filename = g_build_filename (g_get_tmp_dir (), "gegl-magick.png", NULL);
-      cmd = g_strdup_printf ("convert \"%s\"'[0]' \"%s\"",
-                             op_magick_load->path, filename);
-      if (system (cmd) == -1)
+
+      argv[1] = g_strdup_printf ("%s[0]", op_magick_load->path);
+      argv[2] = filename;
+      if (!g_spawn_sync (NULL, argv, NULL, G_SPAWN_DEFAULT, 
+                         NULL, NULL, NULL, NULL, NULL, NULL))
         g_warning ("Error executing ImageMagick convert program");
 
+      g_free (argv[1]);
 
       graph = gegl_node_new ();
       sink = gegl_node_new_child (graph,
@@ -67,7 +70,6 @@ load_cache (GeglProperties *op_magick_load)
       gegl_node_process (sink);
       op_magick_load->user_data = (gpointer) newbuf;
       g_object_unref (graph);
-      g_free (cmd);
       g_free (filename);
     }
 }