kgo: fix bug

Previously, if CommitOffsetsSync was called during a rebalance, and the
context being used is canceled while the rebalance was occurring, then
the client would deadlock.

Internally, committing is blocked if a rebalance is actively happening.
There's some complex logic to have, effectively, a cancelable lock
if the user wants to not wait for a rebalance to complete while trying
to commit offsets.

There was a bug, and since the fix is one line, it's easier to see than
explain.

Closes #668.

pkg/kgo/consumer_group.go

@@ -2584,6 +2584,7 @@ func (g *groupConsumer) commitOffsetsSync(
 
  if err := g.waitJoinSyncMu(ctx); err != nil {
  onDone(g.cl, kmsg.NewPtrOffsetCommitRequest(), kmsg.NewPtrOffsetCommitResponse(), err)
+ close(done)
  return
  }
  unblockCommits := func(cl *Client, req *kmsg.OffsetCommitRequest, resp *kmsg.OffsetCommitResponse, err error) {