xdgmimemagic: Add an assertion to clarify state after hitting an error

This introduces no functional changes, but should help guide static
analysis. Currently, scan-build thinks that `match` could be used after
being freed, because it assumes that `_xdg_mime_magic_parse_error()`
could return any `XdgMimeMagicState`.

See the trace here: https://gnome.pages.gitlab.gnome.org/-/glib/-/jobs/1993739/artifacts/_scan_build/meson-logs/scanbuild/2022-05-10-050755-3977-1/report-6f5f44.html#EndPath

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
2 jobs for parse-error-state in 1 minute and 19 seconds