Correctly handle CK_UNAVAILABLE_INFORMATION

[minego]

The PKCS#11 specification states

The constant CK_UNAVAILABLE_INFORMATION is used in the ulValueLen field to denote an invalid or unavailable value. See C_GetAttributeValue for further details.

This MR exposes the CK_UNAVAILABLE_INFORMATION constant definition (along with CK_EFFECTIVELY_INFINITE which is defined next to it, for completeness sake) and checks for that result after calling C_GetAttributeValue

This corrects a crash when a PKCS#11 module returns this. I believe this will fix the following issues:
#139
#60

Prior to this change the returned value was being interpreted as an actual size, which resulted in attempting to allocate an array of size -1 which causes an overflow:

  File "pkcs11/_pkcs11.pyx", line 726, in pkcs11._pkcs11.Object.__getitem__
  File "pkcs11/_utils.pyx", line 11, in pkcs11._pkcs11.CK_BYTE_buffer
  File "stringsource", line 152, in View.MemoryView.array.__cinit__
OverflowError: Python int too large to convert to C ssize_t

[minego]

On a related note, I ran into this while trying to use this library with the Venafi Code Signing Client PKCS#11 module. I'd love to get that added to the list of supported modules. What is needed to make this happen?

[jrripple]

Nice, I've also been using Venafi and had the same issue. I had a similar solution locally, but I tried out your commits and they fix the issue too.

[minego]

Nice, I've also been using Venafi and had the same issue. I had a similar solution locally, but I tried out your commits and they fix the issue too.

Oh, good! I'm glad to hear that!

I'm one of the developers on the codesigning team at CyberArk (previously Venafi). Thanks for testing the change!