util/iconv: reject improperly packed UTF-8

util/iconv: reject improperly packed UTF-8

If we allow a string that encodes say '\0' as a multi-byte sequence,
we are open to confusion where we mix NUL terminated strings with
sized data blobs, which is to say EVERYWHERE.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14684

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>