BUG/MINOR: http-fetch: Make method smp safe if headers were already forwarded

BUG/MINOR: http-fetch: Make method smp safe if headers were already forwarded

When method sample fetch is called, if an exotic method is found
(HTTP_METH_OTHER), when smp_prefetch_htx() is called, we must be sure the
start-line is still there. Otherwise, HAproxy may crash because of a NULL
pointer dereference, for instance if the method sample fetch is used inside
a unique-id format string. Indeed, the unique id may be generated when the
log message is emitted. At this stage, the request channel is empty.

This patch must be backported as far as 2.0. But the bug exists in all
stable versions for the legacy HTTP mode too. Thus it must be adapted to the
legacy HTTP mode and backported to all other stable versions.