MSVC compiler in debug mode reports accessing to vector[0] element (assertion)
when the vector was empty
(we took only a pointer without dereferencing)
now we are using a dummy char object in such a case
git-svn-id: svn://ttmath.org/publicrep/tito/trunk@398 e52654a7-88a9-db11-a3e9-0013d4bc506e