CVE-2018-10933: Introduced packet filtering

The packet filter checks required states for the incoming packets and
reject them if they arrived in the wrong state.

Fixes T101

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/packet.h

@@ -43,6 +43,12 @@ enum ssh_packet_state_e {
   PACKET_STATE_PROCESSING
 };
 
+enum ssh_packet_filter_result_e {
+    SSH_PACKET_UNKNOWN,
+    SSH_PACKET_ALLOWED,
+    SSH_PACKET_DENIED
+};
+
 int ssh_packet_send(ssh_session session);
 
 SSH_PACKET_CALLBACK(ssh_packet_unimplemented);