src: avoid OOB read in URL parser

This is not a big concern, because right now, all (non-test) inputs
to the parser are `'\0'`-terminated, but we should be future-proof
here and not perform these OOB reads.

PR-URL: https://github.com/nodejs/node/pull/33640


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>