select {
       case <- c:
       case <- c:
}

In this case, c.recvq lists two SudoGs which have the same G.
So we can't use the G as the key to dequeue the correct SudoG,
as that key is ambiguous.  Dequeueing the wrong SudoG ends up
freeing a SudoG that is still in c.recvq.

The fix is to use the actual SudoG pointer as the key.

LGTM=dvyukov
R=rsc, bradfitz, dvyukov, khr
CC=austin, golang-codereviews
https://golang.org/cl/159040043