This change allow to avoid the Visual C++ compiler Warning that occur when you try...

This change allow to avoid the Visual C++ compiler Warning that occur when you try to compare an signed integer with a signed integer (xbt_dynar_length() function returns an unsigned long and the iterators or the indexes are generally integers).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@4980 48e7efb5-ca39-0410-a469-dd3cf9ba447f