Prevent double panic in the Drop of TaksPoolInner

[refnil]

TaskPoolInner would cause failing bevy test (see #1057) to abort the running test suite immediately.

Here is the problematic situation as I understand it:

• The Drop of TaskPoolInner is called on a panicking thread. (I guess that happen when rust is unwinding from a panic)
• The Drop then try to join on all of his threads. However, at least one of them finish with a panic and return an error on the join call.
• except panic when receiving an error.
• Panic in panic happens and everyone is sad (I think?). I'm new to rust so I may be wrong here.
• The program abort.

My idea is to only panic when the current thread is not already panicking.

[cart]

This makes sense to me. Good call!