Refuse assignment of empty strings to line1 proto fields, issue VSL
record LostHdr.

line1 proto fields are request, url, proto and response. status is
numeric and not affected.

Fixes: #781



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5224 d4fa192b-c00b-0410-8231-f00ffab90ce4