GC asserts

[Mark Hamburg <mhamburg@...>]

We have gotten a couple of asserts from the work2 GC.

1. One engineer hit an assert for: g->totalbytes >= g->estimate

2. We've got a couple of cases where the tt value in a table entry doesn't
match the tt value in the object.

Now, as I noted earlier today, we aren't entirely pure on being work2 and
will do some checking ourselves, but it would help to know whether these are
known issues or whether there are particular things we should look for in
trying to debug this or any particular information we should collect to send
back to the experts.

Thanks.
Mark