Fix: memory leak in XML::Node#content=

[toddsundsted]

This PR saves the next pointer before unlinking, as xmlUnlinkNode clears it. In practice, this means the first child is added to the list of unlinked nodes, but all siblings are left off and become leaks.

Sorry for sending these one-by-one. I'm encountering them as I'm stress testing my LibXML extensions. Also, I created this PR against a branch this time, instead of master, hopefully ending the cycle of my nuking the previous PRs.

require "spec"
require "xml"

describe "XML memory management" do
  it "properly unlinks children" do
    xml = %Q|<root>#{"<child/>" * 10}</root>|

    GC.collect
    baseline_rss = get_rss_kb

    1_000_000.times do
      doc = XML.parse(xml)
      root = doc.root.not_nil!
      root.content = "text"
    end

    GC.collect
    final_rss = get_rss_kb
    growth = final_rss - baseline_rss

    p growth: growth

    (growth < 50_000).should be_true
  end
end

private def get_rss_kb : Int64
  {% if flag?(:darwin) %}
    `ps -o rss= -p #{Process.pid}`.strip.to_i64
  {% else %}
    File.read("/proc/self/status").lines.each do |line|
      return line.split[1].to_i64 if line.starts_with?("VmRSS:")
    end
    0_i64
  {% end %}
end

[straight-shoota]

It's great to have them as individual patches 👍
That means each change is clearly isolated and easy to comprehend.