TypeError when OpenSSL library is not found on Windows #316

On Windows, importing bitcoin.core.key can crash with a TypeError
when the OpenSSL library is not found.

This appears to happen because ctypes.util.find_library() may return
None on Windows, which is then passed directly to
ctypes.cdll.LoadLibrary().

Steps to reproduce

On a Windows system without OpenSSL available on PATH:

python -c "import bitcoin.core.key"

I have a small patch that adds an explicit check and raises a clearer error
on Windows instead of crashing.

Would be happy to open a PR if this approach sounds reasonable.