Certificate stack: Remove bogus x509stack_cancel()

Certificate stack: Remove bogus x509stack_cancel()

It seems the #58 and #59 problem is a stray defer separator pop.

The comment above x509stack_cancel() clearly states that the function
should only be called shortly after a x509stack_push(), but there's one
in certificate_traverse() that isn't.

Removing this x509stack_cancel() seems to prevent the crash. I'm still
investigating the original intent of this code.

Tentatively f1xes #58 and #59.