Use NULL rather than 0. (#778)

There was few cases of using literal 0 instead of NULL in the context of
pointers.  While this was a legitimate C code, using NULL rather than 0 makes
the code clearer.

Modules/_csv.c

@@ -365,14 +365,14 @@ dialect_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)
             Py_INCREF(dialect);
         /* Can we reuse this instance? */
         if (PyObject_TypeCheck(dialect, &Dialect_Type) &&
-            delimiter == 0 &&
-            doublequote == 0 &&
-            escapechar == 0 &&
-            lineterminator == 0 &&
-            quotechar == 0 &&
-            quoting == 0 &&
-            skipinitialspace == 0 &&
-            strict == 0)
+            delimiter == NULL &&
+            doublequote == NULL &&
+            escapechar == NULL &&
+            lineterminator == NULL &&
+            quotechar == NULL &&
+            quoting == NULL &&
+            skipinitialspace == NULL &&
+            strict == NULL)
             return dialect;
     }