cparser aborts on enum redefinition

```cpp
enum e
{
    e1
};
enum e
{
    e1
};
```
cparser outputs this :
```
cparser: src/ast/constfold.c:378: determine_enum_values: Assertion `entry->enum_value.tv == NULL || entry->enum_value.tv == tv_next' failed.
Aborted (core dumped)
```