[BUG] clf logs segfault when capturing a non existant header

Hi Willy,

Please find a small patch to prevent haproxy segfaulting when logging captured headers in CLF format.

Example config to reproduce the bug :
listen test :10080
	log 127.0.0.1 local7 debug err
	mode	http
	option	httplog clf
	capture request header NonExistantHeader len 16

--
Cyril Bonté
1 file changed