Avoid useless "++" at the end of functions

Warnings found by the Clang Static Analyzer.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 4361908..309159c 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -6474,7 +6474,7 @@
         }
     }
     /* 0-terminate the output string */
-    *output++ = '\0';
+    *output = '\0';
     Py_XDECREF(exc);
     Py_XDECREF(errorHandler);
     return 0;