Fix PR#22433. The algorithm is_partitioned was testing an item in the middle of the sequence twice.

llvm-svn: 227824
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 9c51284..e8720cc 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -3148,6 +3148,9 @@
     for (; __first != __last; ++__first)
         if (!__pred(*__first))
             break;
+    if ( __first == __last )
+        return true;
+    ++__first;
     for (; __first != __last; ++__first)
         if (__pred(*__first))
             return false;