[3.11] gh-99612: Fix PyUnicode_DecodeUTF8Stateful() for ASCII-only data (GH-99613) (GH-107224)

Previously *consumed was not set in this case.
(cherry picked from commit f08e52ccb0)

Objects/unicodeobject.c

@@ -5120,6 +5120,9 @@ unicode_decode_utf8(const char *s, Py_ssize_t size,
     }
     s += ascii_decode(s, end, PyUnicode_1BYTE_DATA(u));
     if (s == end) {
+        if (consumed) {
+            *consumed = size;
+        }
         return u;
     }