fix bug: BaseNode.RemoveChildren only set the first child as nil

[qiuzhiqian]

if do c.SetNextSibling(nil) and c = c.NextSibling(). After remove the first child the var c is always nil.the next child will not be set nil

[qiuzhiqian]

$ go test -v github.com/yuin/goldmark/ast -run TestRemoveChildren
=== RUN TestRemoveChildren
--- PASS: TestRemoveChildren (0.00s)
ast_test.go:17: &{BaseBlock:{BaseNode:{firstChild: lastChild: parent: next: prev: childCount:0 attributes:[]} blankPreviousLines:false lines:}}
PASS
ok github.com/yuin/goldmark/ast 0.003s