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 (closed), #1462 (closed)). 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.