[libcxx] Error out if __libcpp_mbsrtowcs_l fails in __time_get_storage

If __libcpp_mbsrtowcs_l outputs zero wchar_t's for week days or
month names (due to errors in the locale function setup), these are
matched all the time in __time_get_storage::__analyze, ending up in
an infinite loop, allocating more memory until killed.

Differential Revision: https://reviews.llvm.org/D69553