Ignore pieces with availability of 0

[aritra24]

resolves: #916
Pieces with zero availability shouldn't
consume MaxUnverifiedBytes and shouldn't
be prioritised.

[anacrolix]

Thanks for finding this. The solution isn't quite correct. I am mucking around with a more comprehensive solution at https://github.com/anacrolix/torrent/tree/herp.

Specifically I think it's necessary to ensure requests to individual peers aren't starved by pieces that aren't available to from them.

[aritra24]

Ah that's fair, I wasn't too confident on the solution myself. I can close this pr then. Thanks for helping figure it out ✌🏼

[anacrolix]

Thank you for persisting and finding the issue! I'll have a fix in master shortly.

[anacrolix]

Fixed in 528c5f6