explicitly initialized m_mbs (std::mbstate_t) in utf8 transform iterators.

adjusted tests to be sure that wchar arrays/strings are null terminated.

include/boost/archive/iterators/mb_from_wchar.hpp

@@ -116,6 +116,7 @@ public:
     template<class T>
     mb_from_wchar(T start) :
         super_t(Base(static_cast< T >(start))),
+        m_mbs(std::mbstate_t()),
         m_bend(0),
         m_bnext(0),
         m_full(false)

include/boost/archive/iterators/wchar_from_mb.hpp

@@ -124,8 +124,10 @@ public:
     // make composible buy using templated constructor
     template<class T>
     wchar_from_mb(T start) : 
-        super_t(Base(static_cast< T >(start)))
+        super_t(Base(static_cast< T >(start))),
+        m_mbs(std::mbstate_t())
     {
+        BOOST_ASSERT(std::mbsinit(&m_mbs));
         drain();
     }
     // copy ctor