Nit: Use sane value for NO_PARENT

[milindl]

In the thread implementation, the constant NO_PARENT is defined as -66, please use a less arbitrary value.

As a matter of fact, this weird number was my choice (this wasn't there pre-assignment 1, like a lot of stuff in the threads folder).

[yashsriv]

I thought sir suggested that -_-

[yashsriv]

Closed accidentally