routers/user: ensure that decryption of cookie actually suceeds (#7363)

Previously, only the first return value of ctx.GetSuperSecureCookie
was used to check whether decryption of the auth cookie succeeded.
ctx.GetSuperSecureCookie also returns a second value, a boolean,
indicating success or not. That value should be checked first to
be on the safe side and not rely on internal logic of the encryption
and decryption blackbox.

routers/user/auth.go

@@ -71,8 +71,8 @@ func AutoSignIn(ctx *context.Context) (bool, error) {
 		return false, nil
 	}
 
-	if val, _ := ctx.GetSuperSecureCookie(
-		base.EncodeMD5(u.Rands+u.Passwd), setting.CookieRememberName); val != u.Name {
+	if val, ok := ctx.GetSuperSecureCookie(
+		base.EncodeMD5(u.Rands+u.Passwd), setting.CookieRememberName); !ok || val != u.Name {
 		return false, nil
 	}