Fix sk_deep_copy() implementation

Fix sk_deep_copy() implementation

sk_deep_copy() is bad code. It is less bad than the upstream code, but
still bad: it passes strdup() through a void pointer and assigns it to
a function pointer of different type before calling the latter. That's
not kosher in more than one way.

There is no need for such gymnastics. If we need a deep copy for a type,
we should implement it as appropriate for that type.

Also, we should not expect and even less so allow holes in a STACK_OF().
The only way the vpm->hosts can be populated is by way of this deep_copy
function or x509_param_set_hosts_internal(), which pushes only after a
non-NULL check. Invariants: they're useful.

ok jsing