zlib.c: suppress warning

* ext/zlib/zlib.c (zstream_run): suppress unused-but-set-variable
  warning.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38048 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/zlib/zlib.c

@@ -1086,7 +1086,7 @@ loop:
 
     if (z->stream.avail_in > 0) {
 	zstream_append_input(z, z->stream.next_in, z->stream.avail_in);
-        guard = Qnil; /* prevent tail call to make guard effective */
+        RB_GC_GUARD(guard) = Qnil; /* prevent tail call to make guard effective */
     }
 
     if (args.jump_state)