BUG/MEDIUM: acme: protect against risk of null-deref on connection failure

BUG/MEDIUM: acme: protect against risk of null-deref on connection failure

7 ACME state handlers iterate over hc->res.hdrs, but they can be called
after an error was detected, and the HTTP client will leave res.hdrs NULL
on connection errors before headers are received. Let's check this inside
the loop, like the chkorder handler already does.

Most of them, if not all, need to be backported to 3.2.