Try to fix an order-of-initialisation problem with error_category references.

The symptom, which only occurs in some applications, is a crash due to a
dereference of a null pointer. The exact conditions under which the problem
occurs are not fully understood, so this fix is probably more paranoid than
necessary.


[SVN r41100]

include/boost/system/windows_error.hpp

@@ -107,7 +107,7 @@ namespace boost
     namespace windows_error
     {
       inline error_code make_error_code( windows_error_code e )
-        { return error_code( e, system_category ); }
+        { return error_code( e, get_system_category() ); }
     }
 
   }  // namespace system