Fix DH ASN1 decode so that it detects named groups.

The dh->nid was not being set if the loaded p,g matched an inbuilt named
group for "DH".

NOTE: The "DHX" related path already worked since it calls DH_set0_pqg()
(which does the name group check).

This bug was detected when new tests were added for dh5114 groups, combined
with the no-cache tests i.e. loading+import+export set the nid,
but just loading did not.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14207)
