I fixed some undefined behaviour in string tests in r238609, but I only
fixed the narrow char versions. This applies the same fixes to the
wchar_t ones. These problems were found when testing a patch to make
std::basic_string usable in constexpr.
libstdc++-v3/ChangeLog:
* testsuite/21_strings/basic_string/modifiers/append/wchar_t/1.cc:
Fix reads past the end of strings.
* testsuite/21_strings/basic_string/operations/compare/wchar_t/1.cc:
Likewise.
* testsuite/experimental/string_view/operations/compare/wchar_t/1.cc:
Likewise.
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