Crash in substitution of nested TypeVar after TypeVarTuple

During work on #93330 I have found an error in the C code which causes crash when substitute a TypeVar in nested generic alias after TypeVarTuple.

For example:
```
>>> from typing import *
>>> T = TypeVar('T')
>>> Ts = TypeVarTuple('Ts')
>>> tuple[*Ts, list[T]][int, str, bool]
Segmentation fault (core dumped)
```