sandbox: terminate os_dirent_ls() result list

sandbox: terminate os_dirent_ls() result list

Each node in the linked-list that os_dirent_ls() returns has its next
pointer set only when the next node is created. For the last node in the
list, there is no next node, so this never happens, and the next pointer
is never initialized. Explicitly initialize the next pointer so that it
isn't dangling. Without this, "sb ls" might crash.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>