Fix for error noticed by Olaf van der Spek (ticket #6083)

Added test case for that error

[SVN r75283]

include/boost/lexical_cast.hpp

@@ -1181,7 +1181,7 @@ namespace boost
                 bool const result = !(stream << input).fail();
                 start = stringbuffer.pbase();
                 finish = stringbuffer.pptr();
-                return result;
+                return result && (start != finish);
             }
 
             template <class T>