Fix NULL dereference

Fix NULL dereference when handling REJ packets

src/server.c

@@ -1492,12 +1492,14 @@ static int handle_rej(patty_ax25_server *server,
                       patty_ax25_sock *sock,
                       patty_ax25_frame *frame) {
     unsigned int i,
-                 end = sock->vs;
+                 end;
 
     if (sock == NULL) {
         return 0;
     }
 
+    end = sock->vs;
+
     if (frame->nr > end) {
         end += (sock->mode == PATTY_AX25_SOCK_SABME)? 128: 8;
     }