Fix wrong variable usage in __Pyx_PyIndex_AsSsize_t().

Closes #2616.

Cython/Utility/TypeConversion.c

@@ -376,7 +376,7 @@ static CYTHON_INLINE Py_ssize_t __Pyx_PyIndex_AsSsize_t(PyObject* b) {
     if (sizeof(Py_ssize_t) >= sizeof(long))
         return PyInt_AS_LONG(b);
     else
-        return PyInt_AsSsize_t(x);
+        return PyInt_AsSsize_t(b);
   }
 #endif
   if (likely(PyLong_CheckExact(b))) {