Clear the OpenSSL error queue before cryptohash operations

Clear the OpenSSL error queue before cryptohash operations

Setting up an EVP context for ciphers banned under FIPS generate
two OpenSSL errors in the queue, and as we only consume one from
the queue the other is at the head for the next invocation:

  postgres=# select md5('foo');
  ERROR:  could not compute MD5 hash: unsupported
  postgres=# select md5('foo');
  ERROR:  could not compute MD5 hash: initialization error

Clearing the error queue when creating the context ensures that
we don't pull in an error from an earlier operation.

Discussion: https://postgr.es/m/C89D932C-501E-4473-9750-638CFCD9095E@yesql.se