db: improve formatting of insert statement

db: improve formatting of insert statement

`sql_createstmt` contains a variable `stmt_val` which points to the end
of the SQL already written, where the next chunk should be appended.
Currently, this is assigned after every write:

  sprintf(stmt_val, ...);
  stmt_val = mi->stmt + strlen(mi->stmt);

However, since `sprintf` returns the number of bytes written, increment
`stmt_val` by the return-value of `sprintf` in order to avoid the
repeated `strlen` calls.

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