isakmpd: don't reach into DH in modp_init

[Theo Buehler]

Store prime and generator in intermediate BIGNUMs, then set them on the
DH.  DH_set0_pqg() can't actually fail in this situation, but I prefer
to do error checking mechanically.

There is one more access to dh->pub_key which I will take care of once
we have DH_get0_pub_key() (using DH_get0_key() is too annoying).

Index: dh.c
===================================================================
RCS file: /cvs/src/sbin/isakmpd/dh.c,v
retrieving revision 1.22
diff -u -p -r1.22 dh.c
--- dh.c        13 May 2021 14:28:03 -0000      1.22
+++ dh.c        25 Nov 2021 20:00:46 -0000
@@ -334,14 +334,24 @@ int
 modp_init(struct group *group)
 {
        DH      *dh;
+       BIGNUM  *p = NULL, *g = NULL;
 
        if ((dh = DH_new()) == NULL)
                return (-1);
        group->dh = dh;
 
-       if (!BN_hex2bn(&dh->p, group->spec->prime) ||
-           !BN_hex2bn(&dh->g, group->spec->generator))
+       if (!BN_hex2bn(&p, group->spec->prime) ||
+           !BN_hex2bn(&g, group->spec->generator)) {
+               BN_free(p);
+               BN_free(g);
                return (-1);
+       }
+
+       if (!DH_set0_pqg(dh, p, NULL, g)) {
+               BN_free(p);
+               BN_free(g);
+               return (-1);
+       }
 
        return (0);
 }