(gdb) file opendkim/opendkim
Reading symbols from /opendkim-2.7.0/opendkim/opendkim...done.
(gdb) b opendkim-db.c:5924
Breakpoint 1 at 0x41fcff: file opendkim-db.c, line 5924.
(gdb) r -p inet:9995 -x /etc/opendkim.conf
Starting program: /opendkim-2.7.0/opendkim/opendkim -p inet:9995 -x /etc/opendkim.conf
warning: no loadable sections found in added symbol-file system-supplied DSO at 0x7ffff7ffa000
[Thread debugging using libthread_db enabled]
[New Thread 0x7ffff5bdf700 (LWP 28981)]
[Thread 0x7ffff5bdf700 (LWP 28981) exited]

Breakpoint 1, dkimf_db_walk (db=0x6663c0, first=true, key=0x0, keylen=0x0, req=0x7ffffffea1f0, reqnum=2) at opendkim-db.c:5938
5938                            snprintf(query, sizeof query, "SELECT %s,%s FROM %s",
(gdb) p query
$1 = "SELECT domain_name,id FROM ActiveSigningKeys\000"...

*Executing on my database results in these values:
domain_name        | id
secondaryname.com  |  1
adamjacques.com    |  2
alternatename.com  |  4
Note that there are not any null values in the database.
Continuing...

(gdb) n
5943                            if (err < 0)
(gdb) n
5949                            err = odbx_result((odbx_t *) db->db_handle,
(gdb) n
5951                            if (err < 0)
(gdb) n
5958                            db->db_cursor = result;
(gdb) n
5961                    err = odbx_row_fetch(result);
(gdb) n
5962                    if (err < 0)
(gdb) p result[0]
$3 = {handle = 0x6664f0, generic = 0x68ccc0, aux = 0x686000}
(gdb) n
5970                    if (err == ODBX_RES_DONE)
(gdb) n
5973                    fields = odbx_column_count(result);
(gdb) n
5974                    if (fields == 0)
(gdb) p fields
$4 = 2
(gdb) n
5982                    if (key != NULL && keylen != NULL)
(gdb) n
5989                    if (reqnum != 0)
(gdb) n
5993                            for (c = 0; c < reqnum; c++)
(gdb) n
5995                                    if (c >= fields)
(gdb) p c
$5 = 0
(gdb) p reqnum
$6 = 2
(gdb) n
6004                                                                            c + 1);
(gdb) n
6003                                            val = (char *) odbx_field_value(result,
(gdb) n
6006                                            req[c].dbdata_buflen = strlcpy(req[c].dbdata_buffer,
(gdb) p val
$7 = 0x690cf2 "1"
(gdb) p odbx_field_value(result, c)
$8 = 0x690ce0 "secondaryname.com"
(gdb) n
6008                                                                           req[c].dbdata_buflen);
(gdb) n
6006                                            req[c].dbdata_buflen = strlcpy(req[c].dbdata_buffer,
(gdb) p req[c]
$9 = {dbdata_flags = 0, dbdata_buffer = 0x7ffffffeb770 "", dbdata_buflen = 1024}
(gdb) n
5993                            for (c = 0; c < reqnum; c++)
(gdb) n
5995                                    if (c >= fields)
(gdb) p c
$10 = 1
(gdb) n
6004                                                                            c + 1);
(gdb) n
6003                                            val = (char *) odbx_field_value(result,
(gdb) n
6006                                            req[c].dbdata_buflen = strlcpy(req[c].dbdata_buffer,
(gdb) p val
$11 = 0x0
(gdb) p req[c]
$17 = {dbdata_flags = 0,
  dbdata_buffer = 0x7ffffffec7b0 "nDKIM/table=ActiveSigningKeys?keycol=id?datacol=domain_name,selector,private_key",
  dbdata_buflen = 1024}
(gdb) n
Program received signal SIGSEGV, Segmentation fault.
0x0000000000443702 in strlcpy (
    dst=0x7ffffffec7b0 "nDKIM/table=ActiveSigningKeys?keycol=id?datacol=domain_name,selector,private_key", src=0x0, size=1023)
    at strl.c:65
65              for (i = 0; i < size && (dst[i] = src[i]) != 0; i++)


* And now val is null