Avoid setting gen_type to -1 in dh_gen_common_set_params

Avoid setting gen_type to -1 in dh_gen_common_set_params

gh_gen_type_common_set_params looks up a dh contexts gen_type using
name2id, but if it returns error, we set gctx->gen_type to -1, which
is an invalid value, which may lead to undefined behavior in
future calls, in the event that said future calls preform an operation
of the form;
if (gen_type == <VALID VALUE>) {
        do_stuff
else {
        do_other_stuff
}

Technically it is not correct to continue with the operations on the
gen context after failed parameters setting but this makes it more
predictable.

Fix it by assigning the result of a lookup to a stack variable, and only
update gctx->gen_value if the lookup returns a non-failing value

In leiu of testing this specific case, also add an ossl_assert in dh_gen
to validate the gen_val input prior to continuing, should other code
points attempt to do the same thing

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22991)