Use EVP_PKEY_size() to allocate correct size of signature buffer. (#18)

Do not use fixed buffer size for signature, EVP_SignFinal() requires
buffer for signature at least  EVP_PKEY_size(pkey) bytes in size.

Fixes crash when using 4K RSA signatures (https://github.com/OpenSC/pam_p11/issues/16, https://github.com/OpenSC/pam_p11/issues/15)