Avoid endless loop in case of circular protocol inheritance by adding assertion.
Avoid endless loop in case of circular protocol inheritance by adding assertion.

This commit does not solve the root cause (incomplete circularity detection),
see FIXME.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift