The latter must be seen as undefined behaviour, as it will cause
the `sk_num` function to return -1 to indicate the error, which
causes all kinds of issues.
Thus there now is a panic to abort the program if stacks are initialized
with a null-ptr, and special handling of that case when decoding
a Pkcs file.
The culprit is that `sk_num(stack)` can return -1
as c_int if there is no stack allocated.
Previously, thanks to unsafe casts, this would result in
a isize::max() for len() and iteration size if there was no stack.
Now this case is handled specifically, which fixes the issue.
Sebastian Thiel