[rust] Fix RwHandle failing to wake current task if CLOSED was already received

Test: ran mediasession tests before and after, only after were unstuck.
Unfortunately, a repeatable testcase for this would be extremely
difficult to craft as it is tightly coupled to the async runtime and
zircon port notifications.

Change-Id: I7e3fcf1aa278d61f721d1ebf0c3fe8783fd7f3c9
1 file changed