DFS: Check os_get_random() result

This use does not really need a strong random number, so fall back to
os_random() if a theoretical error case occurs. (CID 72682)

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

src/ap/dfs.c

@@ -440,7 +440,8 @@ dfs_get_valid_channel(struct hostapd_iface *iface,
 	if (num_available_chandefs == 0)
 		return NULL;
 
-	os_get_random((u8 *) &_rand, sizeof(_rand));
+	if (os_get_random((u8 *) &_rand, sizeof(_rand)) < 0)
+		_rand = os_random();
 	chan_idx = _rand % num_available_chandefs;
 	dfs_find_channel(iface, &chan, chan_idx, skip_radar);