fix bug #67955

this fixes a regression from 6c2a8068207a02b3d7ae7416a9967dad0a81e61f 
which caused [bug 67955](https://bugs.php.net/bug.php?id=67955):

`smart_str_appendl` is expecting the length as the length of the string,
but key_length is the byte length of the key, including the 0
terminator.

As such, the cookie name appended to the header would now also include
the 0 terminator of the key name which then would be sent to the server.