nvme-cli: smart-log: Fix memory leak for JSON.

nvme-cli: smart-log: Fix memory leak for JSON.

This patch fixes the following memory leak when smart-log command
executed with JSON option:-
==7725== HEAP SUMMARY:
==7725==     in use at exit: 1,661 bytes in 53 blocks
==7725==   total heap usage: 99 allocs, 46 frees, 6,559 bytes allocated
==7725==
==7725== LEAK SUMMARY:
==7725==    definitely lost: 24 bytes in 1 blocks
==7725==    indirectly lost: 1,637 bytes in 52 blocks
==7725==      possibly lost: 0 bytes in 0 blocks
==7725==    still reachable: 0 bytes in 0 blocks
==7725==         suppressed: 0 bytes in 0 blocks

Fix:-
==8322==
==8322== HEAP SUMMARY:
==8322==     in use at exit: 0 bytes in 0 blocks
==8322==   total heap usage: 99 allocs, 99 frees, 6,559 bytes allocated
==8322==

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@hgst.com>