[xml] crash in xmlSchemaFormatNodeForError when node is document

[Rob Richards <rrichards ctindustries net>]

When validating with a schema against an empty document, 
xmlSchemaFormatNodeForError crashes because the document node, passed in 
from xmlSchemaValidateDoc, does not have node->ns and passes the if 
(node->ns != NULL) test resulting in a crash when node->ns->href is 
accessed for the error message.

Either a node type check needs to be added with the ns test or a case 
for a document node needs to be added to the xmlSchemaFormatNodeForError 
routine. Figure I would leave this up to Kasimier to decide if he wants 
to added specific error message for document or continue to handle it as 
an element.

Rob