The Library Working Group have approved a change to std::for_each_n that
requires it to handle negative N gracefully, which we were not doing for
random access iterators.

	* include/bits/stl_algo.h (for_each_n): Handle negative count.
	* testsuite/25_algorithms/for_each/for_each_n_debug.cc: New test.

From-SVN: r277932