Initialize hapd->nr_db in hostapd_alloc_bss_data()

Initialize hapd->nr_db in hostapd_alloc_bss_data()

Previously, this was initialized in hostapd_setup_bss() which made it
possible for a REMOVE_NEIGHBOR control interface command to be issued
prior to the list head pointers having been set. That resulted in a NULL
pointer dereference. Fix this by initializing the list head at the time
the data structure gets allocated.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>