Fix crash on executing nacl-manifest twice.

gdb/nacl-manifest.c

 /* Handle Native Client manifest files.
 
    Copyright (C) 2011 Free Software Foundation, Inc.
 
    This file is part of GDB.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.
@@ skipping 100 matching lines @@
 
       xfree (nacl_file_list);
       nacl_file_list = next;
     }
 }
 
 static void
 nacl_manifest_free (void)
 {
   nacl_manifest_free_list(nacl_file_list_32);
+  nacl_file_list_32 = NULL;
   nacl_manifest_free_list(nacl_file_list_64);
+  nacl_file_list_64 = NULL;
   xfree (nacl_program_filename_32);
   xfree (nacl_program_filename_64);
   nacl_program_filename_32 = NULL;
   nacl_program_filename_64 = NULL;
 }
 
 /* Very dumb parser for JSON subset used in Native Client manifest files.
    This is a SAX-style parser that runs callbacks on JSON events.  */
 
 struct json_manifest_reader
@@ skipping 291 matching lines @@
 
   c = add_com ("nacl-irt", class_files, nacl_irt_command,
                _("Use FILE as Native Client IRT to be debugged."));
   set_cmd_completer (c, filename_completer);
 
   c = add_com ("nacl-manifest", class_files, nacl_manifest_command,
                _("Use FILE as Native Client manifest for the program"
                  " to be debugged."));
   set_cmd_completer (c, filename_completer);
 }