output: DBI: fix NUL-termination of escaped SQL string

output: DBI: fix NUL-termination of escaped SQL string

On error, `dbi_conn_quote_string_copy` returns zero.  In this case, we
need to set `*dst` to NUL.  Handle a return-value of `2` as normal
below.  `1` is never returned.

Replace `strncpy` with `memcpy`: using `strncpy` is nearly always a
mistake, and we don't need its special behaviour here.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>