GMarkup buffer overflow fixes for error handling, round 2
A follow up to some prior work on avoiding falling off the end of the buffer when formatting parser error messages (see #1461, #1462). This makes it work with non-nul-terminated input strings.
As with the other issues, this isn’t a critical security fix, since nobody should be using
GMarkup with untrusted data; and most ways of loading any data add a trailing nul for paranoia anyway.