wgengine/magicsock: fix buggy fast path in Conn.SetNetworkMap

Spotted by Maisem.

Fixes #6680

Change-Id: I5fdc01de8b006a1c43a2a4848f69397f54b4453a
Co-authored-By: Maisem Ali <maisem@tailscale.com>
Co-authored-By: Andrew Dunham <andrew@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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