Fix theoretical NULL dereference in debug printf

In hostapd_logger_cb(), module_str is checked for NULL (on a path that
does not get executed in practice), and then possibly dereferenced in
snprintf() anyway.

Signed-hostap: Eytan Lifshitz <eytan.lifshitz@intel.com>

hostapd/main.c

@@ -92,7 +92,8 @@ static void hostapd_logger_cb(void *ctx, const u8 *addr, unsigned int module,
 	if (hapd && hapd->conf && addr)
 		os_snprintf(format, maxlen, "%s: STA " MACSTR "%s%s: %s",
 			    hapd->conf->iface, MAC2STR(addr),
-			    module_str ? " " : "", module_str, txt);
+			    module_str ? " " : "", module_str ? module_str : "",
+			    txt);
 	else if (hapd && hapd->conf)
 		os_snprintf(format, maxlen, "%s:%s%s %s",
 			    hapd->conf->iface, module_str ? " " : "",