This patch comes from http://hg.code.sf.net/p/graphicsmagick/code/rev/be898b7c97bd.

diff -ur a/coders/wmf.c b/coders/wmf.c
--- a/coders/wmf.c	2016-09-05 15:20:23.000000000 -0400
+++ b/coders/wmf.c	2017-08-19 10:38:08.984187264 -0400
@@ -2719,8 +2719,8 @@
   if(image->exception.severity != UndefinedException)
     ThrowException2(exception,
                    CoderWarning,
-                   ddata->image->exception.reason,
-                   ddata->image->exception.description);
+                   image->exception.reason,
+                   image->exception.description);
 
   if(logging)
     (void) LogMagickEvent(CoderEvent,GetMagickModule(),"leave ReadWMFImage()");
