OpenSSH: Fix logic error in DisableForwarding option

This option was documented as disabling X11 and agent forwarding but it
failed to do so. Spotted by Tim Rice.

Obtained from: OpenBSD d31ec64016fc
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 3620d70511dc8bf45752028dac0af6f157ec6146)
(cherry picked from commit fcda475ccfcabe6f70e6ef25ccd507ac4b92c1ee)