Fixes#11108.
It only sets q if a valid named group is found.
The function signature was recently changed to pass a non const DH pointer
in order to allow the nid to be cached internally. As an extension of this
the value of q can now also be set as q is always known for named groups.
The length field is also set if q is set.
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11114)
