MAINT: fix another wrong NULL check

silences cppcheck error, not a bug as the only caller checks the python
error.

numpy/core/src/multiarray/ctors.c

@@ -2960,7 +2960,7 @@ _calc_length(PyObject *start, PyObject *stop, PyObject *step, PyObject **next, i
     }
     if (len > 0) {
         *next = PyNumber_Add(start, step);
-        if (!next) {
+        if (!*next) {
             return -1;
         }
     }