BUILD: htx: fix a used uninitialized warning on is_cookie2

gcc-3.4 reports this which actually looks like a valid warning when
looking at the code, it's unsure why others didn't notice it :

src/proto_htx.c: In function `htx_manage_server_side_cookies':
src/proto_htx.c:4266: warning: 'is_cookie2' might be used uninitialized in this function
1 file changed