plug two more leaks in gpt_write

plug two more leaks in gpt_write

* libparted/labels/gpt.c (gpt_write):
1,024 bytes in 2 blocks are definitely lost...
   at 0x4A0739E: malloc (vg_replace_malloc.c:207)
   by 0x416F1B: ped_malloc (libparted.c:270)
   by 0x43E967: pth_get_raw (gpt.c:334)
   by 0x44039D: gpt_write (gpt.c:1067)
   by 0x418F9A: ped_disk_commit_to_dev (disk.c:486)
   by 0x418FE0: ped_disk_commit (disk.c:509)
   by 0x40AF7A: do_mklabel (parted.c:622)
   by 0x40A055: command_run (command.c:139)
   by 0x4121A4: non_interactive_mode (ui.c:1540)