Fix crash in Zenmap DiffViewer

http://seclists.org/nmap-dev/2014/q2/185

zenmap/zenmapGUI/DiffCompare.py

@@ -298,7 +298,7 @@ class ScanChooser(HIGVBox):
     def get_nmap_output(self):
         """Return the currently selected scan's output as a string, or None if
         no valid scan is selected."""
-        return self.parsed_scan
+        return self.parsed_scan.get_nmap_output()
 
     nmap_output = property(get_nmap_output)
     parsed_scan = property(get_parsed_scan)