[xml] xmlPreviousElementSibling mistake

[François Delyon <f delyon satimage fr>]

In tree.c 3530 (libxml2 2.7.6), the function xmlPreviousElementSibling  
ends with:

    while (node != NULL) {
        if (node->type == XML_ELEMENT_NODE)
            return(node);
        node = node->next;
    }

I suggest
        node = node->prev;

François.