Allow for timeout message while waiting for sender

In 9c36c1b0 we fixed counting bug in the bufffer but introduced another
one. We started asserting that the timeout message can only arrive when
the sender is not nil, but in fact it can absolutely happen that the
buffer is asked to deliver a message while one is in flight. This causes
the rexi_buffer to crash. The fix is to simply restore the clause that
ignores the timeout message if the server is waiting for a sender to
return.

BugzID: 32669