Improve CREATE SUBSCRIPTION option parsing

Improve CREATE SUBSCRIPTION option parsing

When creating a subscription with slot_name = NONE, we failed to check
that also create_slot = false and enabled = false were set.  This
created an invalid subscription and could later lead to a crash if a
NULL slot name was accessed.  Add more checks around that for
robustness.

Reported-by: tushar <tushar.ahuja@enterprisedb.com>