The following link explains the issue in detail:
http://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=23117

--------------------------------------------------------------------------------
--- ImageMagick-6.8.3/coders/url.c	Mon Dec 10 05:09:00 2012
+++ ImageMagick-6.8.3/coders/url.c	Mon Apr 22 00:07:50 2013
@@ -154,12 +154,9 @@
     file=fdopen(unique_file,"wb");
   if ((unique_file == -1) || (file == (FILE *) NULL))
     {
-      read_info=DestroyImageInfo(read_info);
-      (void) CopyMagickString(image->filename,read_info->filename,
-        MaxTextExtent);
       ThrowFileException(exception,FileOpenError,"UnableToCreateTemporaryFile",
-        image->filename);
-      image=DestroyImageList(image);
+        read_info->filename);
+      read_info=DestroyImageList(read_info);
       return((Image *) NULL);
     }
   (void) CopyMagickString(filename,image_info->magick,MaxTextExtent);