plug leaks in gpt_write

plug leaks in gpt_write

* libparted/labels/gpt.c (gpt_write):
6,674 bytes in 11 blocks are definitely lost...
  at 0x4A0739E: malloc (vg_replace_malloc.c:207)
  by 0x416F1B: ped_malloc (libparted.c:270)
  by 0x43E76D: pth_new (gpt.c:289)
  by 0x43E7E2: pth_new_zeroed (gpt.c:298)
  by 0x43FF61: _generate_header (gpt.c:970)
  by 0x440434: gpt_write (gpt.c:1074)
  by 0x418F9A: ped_disk_commit_to_dev (disk.c:486)
  by 0x418FE0: ped_disk_commit (disk.c:509)
  by 0x40BB35: do_mkpartfs (parted.c:990)