A receiver could get a spurious empty tube status, due to
receive_tube() racing with send_tube(). See the added comments into
the code for details about the resolution.

At this chance, guard against load/store tearing on shared pointers.

Pending issue: we still have a potential connectivity issue between
the prep and finish ops when pushing to a tube.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>