ELAST should point to the last valid error string value. However, `_sys_nerr` provides the number of elements in the errlist array. Since the index is 0-based, this is off-by-one. Adjust it accordingly. Thanks to David Majnemer for catching this! git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291336 91177308-0d34-0410-b5e6-96231b3b80d8