cmd: gpt: Fix freeing gpt_pte in gpt_verify()

In case when either gpt_verify_headers() or gpt_verify_partitions()
fails, the memory allocated for gpt_pte will be freed in those functions
internally, but gpt_pte will still contain non-NULL dangling pointer.
The attempt to free it in those cases in gpt_verify() leads to "use
after free" error, which leads to a "Synchronous abort" exception.

This issue was found by running the next command on the device with
incorrect partition table:

    => gpt verify mmc 0 $partitions

which results to:

    No partition list provided - only basic check
    "Synchronous Abort" handler, esr 0x96000021, far 0xba247bff
    ....

Fix the issue by only freeing gpt_pte if none of those functions failed.

Fixes: bbb9ffac ("gpt: command: Extend gpt command to support GPT table verification")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>