returns the next key in the list created by a previous op_keylist_start() operation in the context ctx. The key will have one reference for the user.
If the last key in the list has already been returned, op_keylist_next() returns GPG_ERR_EOF.
The function returns the error code GPG_ERR_INV_VALUE if ctx or r_key is not a valid pointer, and GPG_ERR_ENOMEM if there is not enough memory for the operation.