WNM: Fix time_zone advertisement without time_zone configuration
If time_advertisement=2 is included in hostapd configuration, but
time_zone is unset, the previous implementation tried to write the Time
Zone element into management frames. This resulted in segmentation fault
when trying to dereference a NULL pointer. Fix that by skipping addition
of this element when time_zone parameter is not set.
Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>