fix for channel to be closed gracefully

[San9H0]

If you input to closed channel, we face error panic: Input to a closed channel

It actually happened when taskQueue <- task in Submit function in my project

I am using Stopped() function for check. This does not guarantee that the channel is not closed.

and I think most application is N Senders model for channel.

This fix will be necessary

[gammazero]

Consider using recover to catch the "send on closed channle" error. Here is an example: https://github.com/gammazero/workerpool/pull/80/files#. Please also see the comments here for explanation why panic is wanted: #77 (comment)