airo: return from set_wep_key() when key length is zero

Even if keylen == 0 is a bug and should not really happen, better avoid
possibility of passing bad value to firmware.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/airo.c

@@ -5255,7 +5255,8 @@ static int set_wep_key(struct airo_info *ai, u16 index, const char *key,
 	WepKeyRid wkr;
 	int rc;
 
-	WARN_ON(keylen == 0);
+	if (WARN_ON(keylen == 0))
+		return -1;
 
 	memset(&wkr, 0, sizeof(wkr));
 	wkr.len = cpu_to_le16(sizeof(wkr));