ext/zlib/zlib.c: initialize return value

* ext/zlib/zlib.c (zstream_run_func): initialize the return value,
  interrupt flag may set before starting loop.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36341 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/zlib/zlib.c

@@ -924,6 +924,7 @@ zstream_run_func(void *ptr)
     struct zstream *z = args->z;
     uInt n;
 
+    err = Z_OK;
     while (!args->interrupt) {
 	n = z->stream.avail_out;
 	err = z->func->run(&z->stream, flush);