BUG/MINOR: http-fetch: make http_first_req() check for HTTP first

BUG/MINOR: http-fetch: make http_first_req() check for HTTP first

smp_fetch_http_first_req() reads ->txn.http->flags without first
checking if txn.http is properly allocated. In theory if called from
the wrong context it could crash, even though tests where it's called
from "tcp-request content" don't seem to have any effect. Let's fix
it regardless, at least to dissipate the doubt. It can be backported
everywhere.