[nk-batman21]

Siblings, SiblingsFiltered and SiblingsMatcher panic with a nil pointer dereference when the selection contains a node without a parent. This happens when some nodes of a selection are removed and the selection is used again, and when calling them on doc.Selection or on a Clone()d selection.

doc, _ := goquery.NewDocumentFromReader(strings.NewReader(`<ul><li>a</li><li>b</li><li>c</li></ul>`))
items := doc.Find("li")
items.Eq(0).Remove()
items.NextAll().Length() // 1
items.Siblings()         // panic: runtime error: invalid memory address or nil pointer dereference

getSiblingNodes passes n.Parent to getChildrenWithSiblingType, which reads parent.FirstChild in the siblings case. Next, Prev, NextAll and PrevAll already return nothing for these nodes, and jQuery's siblings() returns an empty set for an element without a parent. This change skips nodes without a parent, so items.Siblings() above returns b and c, and doc.Siblings() returns an empty selection.

Ran: go test ./... -v -cover (the new TestSiblingsWithoutParent panics without the change and passes with it)
Ran: go vet ./..., gofmt -l