re PR libstdc++/65049 (Undefined behaviour with std::char_traits<char>)

re PR libstdc++/65049 (Undefined behaviour with std::char_traits<char>)

	PR libstdc++/65049
	* include/bits/char_traits.h (char_traits<char>::compare,
	char_traits<char>::find, char_traits<char>::move,
	char_traits<char>::copy, char_traits<char>::assign): Check for zero
	length.
	(char_traits<wchar_t>::compare, char_traits<wchar_t>::find,
	char_traits<wchar_t>::move, char_traits<wchar_t>::copy,
	char_traits<wchar_t>::assign): Likewise.
	(char_traits<char16_t>::move, char_traits<char16_t>::copy): Likewise.
	(char_traits<char32_t>::move, char_traits<char32_t>::copy): Likewise.
	* include/ext/pod_char_traits.h (char_traits<character<>>::move,
	char_traits<character<>>::copy): Likewise.
	* testsuite/21_strings/char_traits/requirements/char/65049.cc: New.
	* testsuite/21_strings/char_traits/requirements/char16_t/65049.cc:
	New.
	* testsuite/21_strings/char_traits/requirements/char32_t/65049.cc:
	New.
	* testsuite/21_strings/char_traits/requirements/wchar_t/65049.cc:
	New.

From-SVN: r227127