Fix CVE-2016-5118 (popen() shell vulnerability via filename).

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-5118

Upstream patch copied from the bug announcement:
http://seclists.org/oss-sec/2016/q2/432
https://marc.info/?l=oss-security&m=146455222600609&w=2

diff -r 33200fc645f6 magick/blob.c
--- a/magick/blob.c	Sat Nov 07 14:49:16 2015 -0600
+++ b/magick/blob.c	Sun May 29 14:12:57 2016 -0500
@@ -68,6 +68,7 @@
 */
 #define DefaultBlobQuantum  65541
 
+#undef HAVE_POPEN
 
 /*
   Enum declarations.
