GPT: fix memory leaks identified by Coverity

Create a common exit for most of the error handling code in
do_rename_gpt_parts.   Delete the list elements in disk_partitions
before calling INIT_LIST_HEAD from get_gpt_info() a second time.

The SIZEOF_MISMATCH error is not addressed, since that problem was
already fixed by "GPT: incomplete initialization in
allocate_disk_part".

Signed-off-by: Alison Chaiken <alison@peloton-tech.com>
Reported-by: Coverity (CID: 167222, 167235, 167237)
Reviewed-by: Tom Rini <trini@konsulko.com>