ST1015: don't flag default case that is involved in fallthrough

[dominikh]

Adapted from real code seen in the wild:

func fn(x int) {
	switch x {
	case 0:
		println("we can do zero")
	default:
		println("we can't do x, we'll pretend it's 1")
		fallthrough
	case 1:
		println("we can do one")
	}
}

Same issue exists when we fall into the default branch.

Updates #739