ASSERT in updateWithIndirection() that we haven't already updated this
object with an indirection, and fix two places in the RTS where this
could happen.

The problem only occurs when we're in a black-hole-style loop, and
there are multiple update frames on the stack pointing to the same
object (this is possible because of lazy black-holing).  Both stack
squeezing and asynchronous exception raising walk down the stack and
remove update frames, updating their contents with indirections.  If
we don't protect against multiple updates, the mutable list in the old
generation may get into a bogus state.