libstdc++: [_GLIBCXX_DEBUG] Fix std::__niter_base behavior

std::__niter_base is used in _GLIBCXX_DEBUG mode to remove _Safe_iterator<>
wrapper on random access iterators. But doing so it should also preserve 
original
behavior to remove __normal_iterator wrapper.

libstdc++-v3/ChangeLog:

     * include/bits/stl_algobase.h (std::__niter_base): Redefine the 
overload
     definitions for __gnu_debug::_Safe_iterator.
     * include/debug/safe_iterator.tcc (std::__niter_base): Adapt 
declarations.

Ok to commit once all tests completed (still need to check pre-c++11) ?

François