fix: Buffer overflow in bytes_to_read

[BradleyKirton]

Hi there 👋

I encountered a runtime exception in invoke/terminals.py::bytes_to_read on Python 3.14.

I believe the issue is because the original version passed the incorrect sized arg to fcntl.ioctl.

gh-132915: [fcntl.fcntl()](https://docs.python.org/3/library/
fcntl.html#fcntl.fcntl) and fcntl.ioctl() can now detect a buffer overflow and raise SystemError. The stack and memory can be corrupted in such case, so treat this error as fatal.

See python/cpython#132915

I have created this PR which provides correctly sized arg.

[M1troll]

Hey!
I encountered the same issue =(

[stanislaw]

👍

It would be great to have this integrated. My tasks break occasionally with Python 3.14 😢

[matthiask]

I can run any fabric task which doesn't accept input and type into the terminal while the task is running and it crashes with the mentioned buffer overflow.

Applying this patch fixes it locally for me.