guri: Fix paths starting with ‘//’ when parsing

This fixes cases where calling g_uri_to_string() immediately after a successful call to g_uri_parse() would cause an assertion failure for URIs like data:/.// or data:.///.

These appear to be valid URIs. Their paths are normalised to // during parsing as a result of the remove_dot_segments() algorithm. This then falls foul of the restriction from https://datatracker.ietf.org/doc/html/rfc3986#section-3 that

When authority is not present, the path cannot begin with two slash characters ("//").

This is already encoded in an assertion at the top of g_uri_join_internal().

The approach of prefixing the path with /. should be harmless: it guarantees the path starts with /, prevents // being a prefix, and should always be removed by remove_dot_segments() when re-parsing the URI. It’s the same approach as taken in the WhatWG URL spec for a similar (but different) situation: https://url.spec.whatwg.org/#url-serializing (see step 3).

See also: libsoup!415 (79cfd65c)

Signed-off-by: Philip Withnall pwithnall@gnome.org