If we ever add an event, then set it, then add it again, there will be
now two pointers to the event in the event base.  If we delete one and
free it, the first pointer will still be there, and possibly cause a
crash later.

This patch adds detection for this case to the code paths in
eventdns.c, and works around it.  If the warning message ever
displays, then a cleverer fix is in order.

{I am not too confident that this *is* the fix, since bug 957 is very
tricky.  If it is, it is a bugfix on 0.2.0.}