Use traits_type::to_int_type in basic_streambuf<_CharT, _Traits>::xsputn when calling overflow to correctly handle negative signed character types.  This fixes http://llvm.org/bugs/show_bug.cgi?id=14074.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@165884 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/streambuf b/include/streambuf
index e128be5..e34ad23 100644
--- a/include/streambuf
+++ b/include/streambuf
@@ -540,7 +540,7 @@
     {
         if (__nout_ < __eout_)
             *__nout_++ = *__s;
-        else if (overflow(*__s) == __eof)
+        else if (overflow(traits_type::to_int_type(*__s)) == __eof)
             break;
     }
     return __i;