src: fix leading backslash bug in URL

The associated condition mentioned in the URL parsing algorithm of the
WHATWG URL Standard is:
url is special and c is U+005C (\)
So, `special_back_slash` must be updated whenever `special` is updated.

Fixes: https://github.com/nodejs/node/issues/36559

PR-URL: https://github.com/nodejs/node/pull/36613


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>