Xpp3Dom.removeChild(int): fix childMap corruption with duplicate names

[elharo]

Xpp3Dom.removeChild(int) (line 262) calls childMap.values().remove(child) to remove the child from the internal name-to-child map. This is incorrect when multiple children share the same name:

• childMap stores only the last child added with a given name (entries are overwritten by addChild)
• values().remove(child) removes the first matching value — which is the only value for that name in the map
• When removing a child that was overwritten in the map (i.e., a later child with the same name was added), the map entry is left pointing to the wrong child
• When removing the last child with that name, getChild(name) correctly returns null but only by coincidence — the map entry was simply removed

Fix: After removing the child from childList, check if childMap points to that specific child (by reference). If so, scan childList for the last remaining child with the same name and update the map, or remove the entry if none remain.

Fixes #394