tsnet: check a bit harder for https in Server.ListenFunnel

This was mostly already fixed already indirectly in earlier
commits but add a last second length check to this slice so
it can't ever OOB.

Fixes #7860

Change-Id: I31ac17fc93b5808deb09ff34e452fe37c87ddf3a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f