Avoid crash when running task with (*task).run == nil

[tklauser]

The following program currently leads to a crash due to nil pointer
dereference:

wp := workerpool.New(runtime.NumCPU())
defer wp.Close()
if err := wp.Submit("foobar", nil); err != nil {
	panic(err)
}
tasks, err := wp.Drain()

Fix this by checking (*task).run before running it. This way we keep the
semantics of returning a result for every submitted task. The
alternative solution would be to not enqueue the task on
(*WorkerPool).Submit("foobar", nil) and return either nil or an error.
Both would lead to an imbalance between bumber of submitted tasks and
returned results.

[rolinh]

Nice catch!

Fix this by checking (*task).run before running it. This way we keep the
semantics of returning a result for every submitted task.

I like this approach. Running a nil task is the best kind of task to process: not resource intensive and guaranteed to succeed 😄