UBSan: Avoid NULL pointer dereferences on an error path

UBSan: Avoid NULL pointer dereferences on an error path

hapd->conf might be NULL in case initialized failed, so better be
prepared for that when debug printing interface name in the deinit path.

hostapd.c:312:54: runtime error: member access within null pointer of type 'struct hostapd_bss_config'
hostapd.c:351:29: runtime error: member access within null pointer of type 'struct hostapd_bss_config'
hostapd.c:2158:18: runtime error: member access within null pointer of type 'struct hostapd_bss_config'

Signed-off-by: Jouni Malinen <j@w1.fi>