ring: require >=4 points to return true when calling Closed()

[missinglink]

this PR requires a minimum of 4 points for ring.Closed() to return true (as per existing code comment).

note: the current implementation will panic on an empty ring:

panic: runtime error: index out of range [0] with length 0

[missinglink]

I was considering rewriting r[0] == r[len(r)-1] as r[0].Equal(r[len(r)-1]) but I guess it's working fine as-is and probably faster this way 🤷‍♂️

[paulmach]

Thank you!