Improve ISO 8601 parsing by GDateTime
This fixes oss-fuzz issue 16101, avoiding an assertion failure when parsing some malformed ISO 8601 date/time strings.
Note when reviewing: It’s technically an API break in the syntax accepted by g_date_time_new_from_iso8601()
. I think that’s acceptable, but it should be thought about.
/cc @robert.ancell, who originally wrote that code