derp/derphttp: fix nil pointer dereference when closing a netcheck client

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NewNetcheckClient only initializes a subset of fields of derphttp.Client,
and the Close() call added by #5707 was result in a nil pointer dereference.
Make Close() safe to call when using NewNetcheckClient() too.

Fixes #5919

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

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