If s->ctx is NULL then this is an internal error

Coverity was complaining because we checked if s->ctx is NULL and then
later on in the function deref s->ctx anyway. In reality if s->ctx is
NULL then this is an internal error.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/5334)