zlib.c: return unshared string

* ext/zlib/zlib.c (zstream_shift_buffer): create new copied string
  since it cannot be shared ever.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45625 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/zlib/zlib.c

@@ -814,8 +814,7 @@ zstream_shift_buffer(struct zstream *z, long len)
 	return zstream_detach_buffer(z);
     }
 
-    dst = rb_str_subseq(z->buf, 0, len);
-    rb_obj_reveal(dst, rb_cString);
+    dst = rb_str_new(RSTRING_PTR(z->buf), len);
     z->buf_filled -= len;
     memmove(RSTRING_PTR(z->buf), RSTRING_PTR(z->buf) + len,
 	    z->buf_filled);