Allow use of c_str() on wstring.

headers/cpp/std/bastring.h

@@ -350,7 +350,10 @@ private:
 
 public:
   const charT* c_str () const
-    { if (length () == 0) return ""; terminate (); return data (); }
+    {
+	  static const charT null_str[1] = {0};
+	  if (length () == 0) return null_str; terminate (); return data ();
+	}
   void resize (size_type n, charT c);
   void resize (size_type n)
     { resize (n, eos ()); }