nvme-cli: id-ns: Fix memory leak for JSON.

nvme-cli: id-ns: Fix memory leak for JSON.

This patch fixes the following memory leak when id-ns command
executed with JSON option:-
==7642==
==7642== HEAP SUMMARY:
==7642==     in use at exit: 3,297 bytes in 120 blocks
==7642==   total heap usage: 182 allocs, 62 frees, 9,204 bytes allocated
==7642==
==7642== LEAK SUMMARY:
==7642==    definitely lost: 24 bytes in 1 blocks
==7642==    indirectly lost: 3,273 bytes in 119 blocks
==7642==      possibly lost: 0 bytes in 0 blocks
==7642==    still reachable: 0 bytes in 0 blocks
==7642==         suppressed: 0 bytes in 0 blocks
==7642== Rerun with --leak-check=full to see details of leaked memory
==7642==

Fix:-
==7949==
==7949== HEAP SUMMARY:
==7949==     in use at exit: 0 bytes in 0 blocks
==7949==   total heap usage: 182 allocs, 182 frees, 9,204 bytes allocated
==7949==

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