Update test_misc.cpp (#97768)
Potential null dereference after dynamic cast was found during static analysis.
**Description:**
Dereference of `ctx` is performed in `TORCH_CHECK` on line 1176, while `ctx` pointer may equal `nullptr`.
Previous `TORCH_CHECK` on line 1175 checks the value of `ctx_ptr` pointer that may be of type that cannot be casted to `TestContext*`. In such case, `dynamic_cast` returns `nullptr` despite `ctx_ptr` is not equal to `nullptr`.
**Fix:**
- Check `ctx` instead of `ctx_ptr` for equality to zero.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97768
Approved by: https://github.com/kit1980
