Set max character buffer len to 512 per MS specification

The MS specs determine that the character buffer lenngths
for usernames, domains, passwords, alternate shells, etc
can be up to 512 characters including the mandatory null
terminator.

xup/xup.h

@@ -24,6 +24,7 @@
 #include "os_calls.h"
 #include "defines.h"
 #include "xrdp_client_info.h"
+#include "xrdp_constants.h"
 #include "xrdp_rail.h"
 
 #define CURRENT_MOD_VER 4
@@ -154,8 +155,8 @@ struct mod
   int height;
   int bpp;
   int sck_closed;
-  char username[256];
-  char password[256];
+  char username[INFO_CLIENT_MAX_CB_LEN];
+  char password[INFO_CLIENT_MAX_CB_LEN];
   char ip[256];
   char port[256];
   int shift_state;