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.h

@@ -118,6 +118,7 @@ int xml_attribute(const char *name, const char *fmt, ...) __attribute__ ((format
 int xml_newline();
 
 int xml_depth();
+bool xml_tag_open();
 bool xml_root_written();
 
 #endif