libstdc++: More fixes for null pointers used with std::char_traits

libstdc++: More fixes for null pointers used with std::char_traits

The std::char_traits member functions require that [p,p+n) is a valid
range, which is true for p==nullptr iff n==0. But we must not call
memcpy, memset etc, in that case, as they require non-null pointers even
when n==0.

This std::char_traits<char> and std::char_traits<wchar_t> explicit
specializations are already correct, but the primary template has some
bugs.

libstdc++-v3/ChangeLog:

	* include/bits/char_traits.h (char_traits::copy): Return without
	using memcpy if n==0.
	(char_traits::assign): Likewise for memset.