All the little functions created by the IMPLEMENT_STACK_OF() macro will

cast their type-specific STACK into a real STACK and call the underlying
sk_*** function. The problem is that if the STACK_OF(..) parameter being
passed in has a "const *" qualifier, it is discarded by the cast.

I'm currently implementing a fix for this but in the mean-time, this is
one case I noticed (a few type-specific sk_**_num() functions pass in
const type-specific stacks). If there are other errors in the code where
consts are being discarded, we will similarly not notice them. yuck.