fix(bitswap/network): return when context is done

@jbenet @whyrusleeping

This bug (missing return) could tie up the client worker and cause
operations to come to a halt.

network/ipfs_impl.go

@@ -107,6 +107,7 @@ func (bsnet *impl) FindProvidersAsync(ctx context.Context, k util.Key, max int)
 			bsnet.host.Peerstore().AddAddresses(info.ID, info.Addrs)
 			select {
 			case <-ctx.Done():
+				return
 			case out <- info.ID:
 			}
 		}