ra* segfault with zero width column size

[elof2 at sentor.se]

I accidentally set the column size to zero:
   ra -Zb -nnn -r argus.log -s stime:0 proto saddr sport dir:3 daddr:50 dport sbytes dbytes state:13
and got:
   ra[11222]: segfault at 0 ip 0000000000433f3c sp 00007fff01444370 error 4 in ra[400000+9d000]
   Segmentation fault: 11 (core dumped)

Tested on ra versions 3.0.6.2 and 3.0.5.11. Both segfault.

/Elof