Fix unitialized/unterminated malloc'd string

Class documentation without Description, See Also et. al.
resulted in failures on Win64 (and undefined behavior on other
platforms) because a malloc'd string remained improperly terminated
in that case.

Wrapping/vtkWrapPython.c

@@ -2640,6 +2640,7 @@ static void vtkWrapPython_ClassDoc(
 
   comment = (char *)malloc(n);
   cp = comment;
+  *cp = '\0';
 
   if (data->Description)
     {