output: SQLITE3: improve formatting of insert statement

output: SQLITE3: improve formatting of insert statement

`sqlite3_createstmt` contains a variable `stmt_pos` 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_pos, ...);
  stmt_pos = priv->stmt + strlen(priv->stmt);

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

Pablo mangled this original patch to add this chunk at the end of this
patch (originally submitted as a conversion to use strcpy).

+       for (i = 0; i < cols - 1; i++)
+               stmt_pos += sprintf(stmt_pos, "?,");

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