If the user calls Node::find with an invalid XPath query (e.g. "//"), the
library will try to dereference a NULL pointer and cause a SIGSEGV.  The
offending code is here:

NodeSet Node::find(const std::string& xpath) const
{
  xmlXPathContext* ctxt = xmlXPathNewContext(impl_->doc);
  ctxt->node = impl_;
  xmlXPathObject* result = xmlXPathEval((const xmlChar*)xpath.c_str(), ctxt);

  if (result->type != XPATH_NODESET)  /* BOOM! */
  {
    xmlXPathFreeObject(result);
    xmlXPathFreeContext(ctxt);
    throw internal_error("sorry, only nodeset result types supported for now.");


The result of xmlPathEval should be checked before being dereferenced.  I would
recommend an exception be thrown when it is 0.