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.