nvme-print-json: Fix uninitialized variable

nvme-print-json: Fix uninitialized variable

../nvme-print-json.c:1340:22: warning: variable 'node' is uninitialized when used here [-Wuninitialized]
 1340 |         list_for_each(list, node, node) {
      |                             ^~~~
../ccan/ccan/list/list.h:524:40: note: expanded from macro 'list_for_each'
  524 |         list_for_each_off(h, i, list_off_var_(i, member))
      |                                               ^
../ccan/ccan/list/list.h:824:21: note: expanded from macro 'list_off_var_'
  824 |         (container_off_var(var, member) +               \
      |                            ^~~
../ccan/ccan/container_of/container_of.h:142:19: note: expanded from macro 'container_off_var'
  142 |         ((const char *)&(var)->member - (const char *)(var))
      |                          ^~~
../ccan/ccan/list/list.h:738:34: note: expanded from macro 'list_for_each_off'
  738 |         list_for_each_off_dir_((h),(i),(off),next)
      |                                         ^~~
../ccan/ccan/list/list.h:693:9: note: expanded from macro 'list_for_each_off_dir_'
  693 |                                    (off));                              \
      |                                     ^~~
../nvme-print-json.c:1338:31: note: initialize the variable 'node' to silence this warning
 1338 |         nvme_effects_log_node_t *node;
      |                                      ^
      |                                       = NULL

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