gh-104645: fix error handling in marshal tests

Modules/_testcapimodule.c

@@ -1808,10 +1808,9 @@ pymarshal_write_long_to_file(PyObject* self, PyObject *args)
     }
 
     PyMarshal_WriteLongToFile(value, fp, version);
+    assert(!PyErr_Occurred());
 
     fclose(fp);
-    if (PyErr_Occurred())
-        return NULL;
     Py_RETURN_NONE;
 }
 
@@ -1834,10 +1833,9 @@ pymarshal_write_object_to_file(PyObject* self, PyObject *args)
     }
 
     PyMarshal_WriteObjectToFile(obj, fp, version);
+    assert(!PyErr_Occurred());
 
     fclose(fp);
-    if (PyErr_Occurred())
-        return NULL;
     Py_RETURN_NONE;
 }
 
@@ -1895,48 +1893,46 @@ pymarshal_read_long_from_file(PyObject* self, PyObject *args)
 static PyObject*
 pymarshal_read_last_object_from_file(PyObject* self, PyObject *args)
 {
-    PyObject *obj;
-    long pos;
     PyObject *filename;
-    FILE *fp;
-
     if (!PyArg_ParseTuple(args, "O:pymarshal_read_last_object_from_file", &filename))
         return NULL;
 
-    fp = _Py_fopen_obj(filename, "rb");
+    FILE *fp = _Py_fopen_obj(filename, "rb");
     if (fp == NULL) {
         PyErr_SetFromErrno(PyExc_OSError);
         return NULL;
     }
 
-    obj = PyMarshal_ReadLastObjectFromFile(fp);
-    pos = ftell(fp);
+    PyObject *obj = PyMarshal_ReadLastObjectFromFile(fp);
+    long pos = ftell(fp);
 
     fclose(fp);
+    if (obj == NULL) {
+        return NULL;
+    }
     return Py_BuildValue("Nl", obj, pos);
 }
 
 static PyObject*
 pymarshal_read_object_from_file(PyObject* self, PyObject *args)
 {
-    PyObject *obj;
-    long pos;
     PyObject *filename;
-    FILE *fp;
-
     if (!PyArg_ParseTuple(args, "O:pymarshal_read_object_from_file", &filename))
         return NULL;
 
-    fp = _Py_fopen_obj(filename, "rb");
+    FILE *fp = _Py_fopen_obj(filename, "rb");
     if (fp == NULL) {
         PyErr_SetFromErrno(PyExc_OSError);
         return NULL;
     }
 
-    obj = PyMarshal_ReadObjectFromFile(fp);
-    pos = ftell(fp);
+    PyObject *obj = PyMarshal_ReadObjectFromFile(fp);
+    long pos = ftell(fp);
 
     fclose(fp);
+    if (obj == NULL) {
+        return NULL;
+    }
     return Py_BuildValue("Nl", obj, pos);
 }