Don't increment environment pointer past the end.

This should not effect correct programs, since once the
iterator itself is past-the-end, it should not matter what
the underlying state of that iterator is. But, the new behaviour
is more obvious.

Author: vprus

include/boost/program_options/environment_iterator.hpp

@@ -40,8 +40,9 @@ namespace boost {
                 assert(n != s.npos);
                 value().first = s.substr(0, n);
                 value().second = s.substr(n+1);
-            }            
-            ++m_environment;
+                
+                ++m_environment;
+            }
         }
 
     private: