nbft: Fix uninitialized variable

nbft: Fix uninitialized variable

../nbft.c:106:28: warning: variable 'entry' is uninitialized when used here [-Wuninitialized]
  106 |         list_for_each(&nbft_list, entry, 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));                              \
      |                                     ^~~
../nbft.c:90:31: note: initialize the variable 'entry' to silence this warning
   90 |         struct nbft_file_entry *entry;
      |                                      ^
      |                                       = NULL

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