plug a leak in gpt_read

plug a leak in gpt_read

* libparted/labels/gpt.c (gpt_read):
7,195 bytes in 13 blocks are definitely lost...
   at 0x4A0739E: malloc (vg_replace_malloc.c:207)
   by 0x416F1B: ped_malloc (libparted.c:270)
   by 0x43E759: pth_new (gpt.c:287)
   by 0x43E83C: pth_new_from_raw (gpt.c:310)
   by 0x43F329: _read_header (gpt.c:627)
   by 0x43FB31: gpt_read (gpt.c:826)
   by 0x41882B: ped_disk_new (disk.c:210)
   by 0x40B773: do_mkpartfs (parted.c:884)
   by 0x40A055: command_run (command.c:139)