bpo-33231: Fix potential leak in normalizestring() (GH-6386)

(cherry picked from commit 0c1c4563)
Co-authored-by: INADA Naoki <methane@users.noreply.github.com>

Misc/NEWS.d/next/Core and Builtins/2018-04-05-22-20-44.bpo-33231.3Jmo0q.rst

+Fix potential memory leak in ``normalizestring()``.

Python/codecs.c

@@ -78,8 +78,6 @@ PyObject *normalizestring(const char *string)
     }
     p[i] = '\0';
     v = PyUnicode_FromString(p);
-    if (v == NULL)
-        return NULL;
     PyMem_Free(p);
     return v;
 }