output: MYSQL: improve mapping of DB columns to input-keys

output: MYSQL: improve mapping of DB columns to input-keys

Currently, we copy the column-name to a buffer, iterate over it to
replace the underscores with full-stops, using `strchr` from the start
of the buffer on each iteration, then copy the buffer to the input-key's
`name` member.

Apart from the inefficiency, `strncpy` is used to do the copies, which
leads gcc to complain:

  ulogd_output_MYSQL.c:149:17: warning: `strncpy` output may be truncated copying 31 bytes from a string of length 31

Furthermore, the buffer is not initialized, which means that there is
also a possible buffer overrun if the column-name is too long, since
`strncpy` will not append a NUL.

Instead, copy the column-name directly to the input-key using
`snprintf`, and run `strchr` from the last underscore on each iteration.

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