Fixed instance names in tunnel validation methods.
Fixed instance names in tunnel validation methods.

This commit fixes a typo introduced in 9dd68a8 that used an incorrect
instance variable name in Tunnel.EnsureValid. It also makes the instance
variable name more idiomatic in TunnelHostCredentials.EnsureValid.

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen