fix the null pointer dereference

[QiuYitai]

The NULL Dereference vulnerability happens in static void deparseAlterIdentityColumnOptionList (), src/postgres_deparse.c
How the NULL Pointer Dereference happens:

1. When strcmp(def_elem->defname, "generated") == 0 and def_elem->arg == NULL
2. NULL dereference of variable def_elem->arg happens at (castNode(Integer, v)->ival)

static void deparseAlterIdentityColumnOptionList(StringInfo str, List *l)
{
    ListCell *lc = NULL;
    foreach (lc, l)
    {
        DefElem *def_elem = castNode(DefElem, lfirst(lc));
        ......
        else if (strcmp(def_elem->defname, "generated") == 0)
        {
            appendStringInfoString(str, "SET GENERATED ");
=>          if (intVal(def_elem->arg) == ATTRIBUTE_IDENTITY_ALWAYS)
                appendStringInfoString(str, "ALWAYS");
            ......
        }
    }
    ......
}

=>  #define intVal(v)       (castNode(Integer, v)->ival)