libstdc++: Fix range access for empty std::valarray [PR103022]
The std::begin and std::end overloads for std::valarray are defined in
terms of std::addressof(v[0]) which is undefined for an empty valarray.
libstdc++-v3/ChangeLog:
PR libstdc++/103022
* include/std/valarray (begin, end): Do not dereference an empty
valarray. Add noexcept and [[nodiscard]].
* testsuite/26_numerics/valarray/range_access.cc: Check empty
valarray. Check iterator properties. Run as well as compiling.
* testsuite/26_numerics/valarray/range_access2.cc: Likewise.
* testsuite/26_numerics/valarray/103022.cc: New test.