PR libstdc++/89446 fix null pointer dereference in char_traits

PR libstdc++/89446 fix null pointer dereference in char_traits

	PR libstdc++/89446
	* include/bits/char_traits.h (__constant_char_array): Check index is
	in range before dereferencing.
	(char_traits<char>::compare, char_traits<char>::find)
	(char_traits<char8_t>::compare, char_traits<char8_t>::find): Return
	immediately if n is zero.
	(char_traits<wchar_t>::compare, char_traits<wchar_t>::find): Likewise.
	Remove workarounds for PR 67026.
	* testsuite/21_strings/basic_string_view/operators/char/89446.cc:
	New test.
	* testsuite/21_strings/basic_string_view/operators/wchar_t/89446.cc:
	New test.

From-SVN: r269148