The previous parsing code could read off the end of a URI if it had an
incorrect %-escaped character in.

Fix that, and more closely implement parsing for the syntax defined in
RFC 6874, which is the amendment to RFC 3986 which specifies zone ID
syntax.

This requires reworking some network-address tests, which were
previously treating zone IDs incorrectly.

oss-fuzz#23816

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>