Fix a mismerge in kex null-pointer checks.

I removed a vital line of code while fixing the merge conflicts when
cherry-picking 1eb578a488 as
26fe1e26c0, causing Diffie-Hellman key
exchange to be completely broken because the server's host key was
never constructed to verify the signature with. Reinstate it.

ssh.c

@@ -6672,6 +6672,7 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
             bombout(("unable to parse key exchange reply packet"));
             crStopV;
         }
+        s->hkey = ssh->hostkey->newkey(s->hostkeydata, s->hostkeylen);
         s->f = ssh2_pkt_getmp(pktin);
         if (!s->f) {
             bombout(("unable to parse key exchange reply packet"));