Minor cleanup and simplification in dsa_pub_encode()

Minor cleanup and simplification in dsa_pub_encode()

This function has a weird dance of allocating an ASN1_STRING in an
inner scope and assigning it to a void pointer in an outer scope for
passing it to X509_PUBKEY_set0_param() and ASN1_STRING_free() on error.
This can be simplified and streamlined.

ok inoguchi