tool: ifwitool: The function localtime() can return NULL.

This will cause the local_time pointer is passed as the 4th argument
 to function strftime() to also point to NULL. This result in a
 segmentation fault. Thus, it's necessary to add a check of the local_time
 pointer to NULL.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Mikhail Ilin <ilin.mikhail.ol@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>