Update zlib from 1.2.11 to 1.2.12

fix #2439

Because there is a CVE issue impact zlib before 1.2.12,
update zlib to avoid any related problems.

Please check the link below for detailed CVE information:
https://access.redhat.com/security/cve/CVE-2018-25032

test/bench/zlib/inflate_stream.cpp

@@ -16,7 +16,7 @@
 #include <random>
 #include <string>
 
-#include "zlib-1.2.11/zlib.h"
+#include "zlib-1.2.12/zlib.h"
 
 namespace boost {
 namespace beast {