hcrypto PKCS#11 backend: allow digest update with NULL (#378)

In the PKCS#11 backend, add an assertion check to p11_md_update()
to validate that the data length is zero if the data is NULL.

lib/hcrypto/evp-pkcs11.c

@@ -387,6 +387,7 @@ p11_md_update(EVP_MD_CTX *ctx, const void *data, size_t length)
     CK_RV rv;
 
     assert(p11_module != NULL);
+    assert(data != NULL || length == 0);
 
     rv = p11_module->C_DigestUpdate(p11ctx->hSession,
                                     data ? (CK_BYTE_PTR)data : (CK_BYTE_PTR)"",