Philip Withnall authored Jul 01, 2020 and Claudio Saavedra committed Jul 02, 2020

There are two instances in `SoupURI` where `g_ascii_isxdigit()` is
called two bytes ahead of the read pointer to check if a %-encoding is
valid. This is fine when the string being parsed is nul-terminated (as
the first `g_ascii_isxdigit()` call will safely return `FALSE`), but
will result in a read off the end of the buffer if it’s
length-terminated (and doesn’t happen to also be nul-terminated).

Thankfully, that’s not the case in any of the code paths in `SoupURI`
leading to these two instances, so this is not a security issue.

However, the functions should probably be fixed to do an appropriate
length check, just in case they get called from somewhere else in
future.

Spotted by oss-fuzz in oss-fuzz#23815 and oss-fuzz#23818, when it was
fuzzing the new `GUri` implementation in GLib, which is heavily based
off this code.

Includes two unit tests which don’t actually trigger the original
failure (as all strings passed into `SoupURI` are forced to be
nul-terminated), but would trigger it if the nul termination was not
present.

Signed-off-by: Philip Withnall <withnall@endlessm.com>