server: return SSH_OK for ignored SSH_MSG_KEXDH_INIT case

Return SSH_OK for the case that an incoming SSH_MSG_KEXDH_INIT should be
ignored.  That is, for the case that the initial 'first_kex_packet_follows'
guess is incorrect.  Before this change sessions served with libssh can be
observed to error out unexpectedly early when testing with dropbear clients
that send an incompatible guess.

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/server.c

@@ -187,6 +187,7 @@ SSH_PACKET_CALLBACK(ssh_packet_kexdh_init){
     SSH_LOG(SSH_LOG_RARE, "first_kex_packet_follows guess was wrong, "
                           "ignoring first SSH_MSG_KEXDH_INIT message");
     session->first_kex_follows_guess_wrong = 0;
+    rc = SSH_OK;
     goto error;
   }