json: Fix uninitialized variables

json: Fix uninitialized variables

In file included from ../src/nvme/json.c:17:
In function ‘freep’,
    inlined from ‘json_export_nvme_tls_key’ at ../src/nvme/json.c:70:24:
../src/nvme/cleanup.h:24:9: warning: ‘tls_str’ may be used uninitialized [-Wmaybe-uninitialized]
   24 |         free(*(void **)p);
      |         ^~~~~~~~~~~~~~~~~
../src/nvme/json.c: In function ‘json_export_nvme_tls_key’:
../src/nvme/json.c:70:38: note: ‘tls_str’ was declared here
   70 |                 _cleanup_free_ char *tls_str;
      |                                      ^~~~~~~

In function ‘freep’,
    inlined from ‘json_export_nvme_tls_key’ at ../src/nvme/json.c:66:32:
../src/nvme/cleanup.h:24:9: warning: ‘key_data’ may be used uninitialized [-Wmaybe-uninitialized]
   24 |         free(*(void **)p);
      |         ^~~~~~~~~~~~~~~~~
../src/nvme/json.c: In function ‘json_export_nvme_tls_key’:
../src/nvme/json.c:66:39: note: ‘key_data’ was declared here
   66 |         _cleanup_free_ unsigned char *key_data;
      |                                       ^~~~~~~~

In function ‘freep’,
    inlined from ‘json_import_nvme_tls_key’ at ../src/nvme/json.c:37:32,
    inlined from ‘json_update_attributes’ at ../src/nvme/json.c:147:3,
    inlined from ‘json_parse_port’ at ../src/nvme/json.c:177:2,
    inlined from ‘json_parse_subsys’ at ../src/nvme/json.c:212:4,
    inlined from ‘json_parse_host’ at ../src/nvme/json.c:246:4,
    inlined from ‘json_read_config’ at ../src/nvme/json.c:316:4:
../src/nvme/cleanup.h:24:9: warning: ‘key_data’ may be used uninitialized [-Wmaybe-uninitialized]
   24 |         free(*(void **)p);
      |         ^~~~~~~~~~~~~~~~~
../src/nvme/json.c: In function ‘json_read_config’:
../src/nvme/json.c:37:39: note: ‘key_data’ was declared here
   37 |         _cleanup_free_ unsigned char *key_data;
      |                                       ^~~~~~~~

Signed-off-by: Tomas Bzatek <tbzatek@redhat.com>