afsql: Fixed memleak in `afsql_dd_insert_db()`.

When a transaction failed we just returned and leaked all
the allocated resources.

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>
Merge pull request #343 from lbudai/3.6/f/fix-afsql-memleak

afsql: Fixed memleak in `afsql_dd_insert_db()`.