Fix bsup deadlock

[mattnibs]

Closes #6458

[nwt]

Since the goroutine created by scanner.start closes resultChCh when it exits, you should be able to do this instead of adding another select statement.

Suggested change

	case ch := <-s.resultChCh:
	case ch, ok := <-s.resultChCh:
	if !ok {
	return nil, s.ctx.Err()
	}

[mattnibs]

Good point, let's try it

[philrz]

I tested this branch on its latest commit ce285ec using the repro recipe from #6458 and did not trigger any deadlocks so this is a functional 👍 for me.