db: fix back-log capacity checks

db: fix back-log capacity checks

Hitherto, when adding queries to the back-log, the memory usage has been
incremented and decremented by the size of the query structure and the
length of the SQL statement, `sizeof(struct db_stmt) + len`.  However,
when checking whether there is available capacity to add a new query,
the struct size has been ignored.  Amend the check to include the struct
size, and also account for the NULL that terminates the SQL.

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