[mod_magnet] fix regression in lighty.stat (fixes #2877)

fix regression in mod_magnet lighty.stat introduced in lighttpd 1.4.49
in commit commit:b1df38ab

x-ref:
  "lighty.stat failure"
  https://redmine.lighttpd.net/issues/2877

src/mod_magnet.c

@@ -334,14 +334,16 @@ static int magnet_stat(lua_State *L) {
 	handler_t res;
 
 	res = stat_cache_get_entry(srv, con, sb, &sce);
-	stat_cache_content_type_get(srv, con, sb, sce);
-	buffer_free(sb);
 
 	if (HANDLER_GO_ON != res) {
+		buffer_free(sb);
 		lua_pushnil(L);
 		return 1;
 	}
 
+	stat_cache_content_type_get(srv, con, sb, sce);
+	buffer_free(sb);
+
 	lua_newtable(L); // return value
 
 	lua_pushboolean(L, S_ISREG(sce->st.st_mode));