C_Digest does not check if buffer too small before update. Issue #327

C_Digest will now query for the buffer size using sc_pkcs15_md_final
before calling sc_pkcs15_md_update. This avoids doing a double update
when the user passes in a buffer to small, then gets the buffer and calls
C_Digest again.
Merge pull request #330 from dengert/pkcs11-buf-too-small-simple

C_Digest does not check if buffer too small before update. Issue #327