Close an open start tag before a fatal error.

This is part of the error handler that attempts to close open XML tags
at a fatal error. The case wasn't handled where a fatal error happened
while a start tag was still open (didn't have its '>' written yet). This
came from a bug report from David Hingos.

http://seclists.org/nmap-dev/2012/q1/514

xml.cc

@@ -412,6 +412,11 @@ int xml_depth() {
   return xml.element_stack.size();
 }
 
+/* Return true iff a root element has been started. */
+bool xml_tag_open() {
+  return xml.tag_open;
+}
+
 /* Return true iff a root element has been started. */
 bool xml_root_written() {
   return xml.root_written;