Fix Memory Leaks

[Zelmoghazy]

void XML_free(XMLNode *node)
{
    for (int i = 0; i < node->children.size; i++) {
        XML_free(node->children.data[i]);
    }
    free(node->children.data);
    XMLNode_free(node);
}

The XMLDocument_free function doesnt really free the XML tree it only frees the root node, I think this frees the entire tree tested with valgrind although relying on recursion is not a good idea an iterative version is more complicated.

[calum904]

@Zelmoghazy I have addressed this on a forked repository pending PR.