Protect against crash on too small SSH_MSG_IGNORE packets.

Reported by Bob Alexander <balexander@expressor-software.com>
in <http://thread.gmane.org/gmane.network.ssh.libssh2.devel/2530>.

src/packet.c

@@ -533,11 +533,15 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
             break;
 
         case SSH_MSG_IGNORE:
-            /* As with disconnect, back it up one and add a trailing NULL */
-            memcpy(data + 4, data + 5, datalen - 5);
-            data[datalen] = '\0';
-            if (session->ssh_msg_ignore) {
-                LIBSSH2_IGNORE(session, (char *) data + 4, datalen - 5);
+            if (datalen >= 5) {
+                /* Back it up one and add a trailing NULL */
+                memmove(data, data + 1, datalen - 1);
+                data[datalen] = '\0';
+                if (session->ssh_msg_ignore) {
+                    LIBSSH2_IGNORE(session, (char *) data + 4, datalen - 1);
+                }
+            } else if (session->ssh_msg_ignore) {
+                LIBSSH2_IGNORE(session, "", 0);
             }
             LIBSSH2_FREE(session, data);
             session->packAdd_state = libssh2_NB_state_idle;