scanner: Plug two memory leaks

Found with both ASan leak sanitizer and Valgrind. We were trivially
leaking the enum name for every arg parsed by the scanner which had one.
If libxml-based DTD validation was enabled, we would also leak the DTD
itself, despite diligently freeing the document, context, etc.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>