fiber: don't crash on wakeup with dead fibers

When fiber has finished its work it ended up in two cases:
1) If no "joinable" attribute set then the fiber is
   simply recycled
2) Otherwise it continue hanging around waiting to be
   joined.

Our API allows to call fiber_wakeup() for dead but joinable
fibers (2) in release builds without any side effects, such
fibers are simply ignored, in turn for debug builds this
causes assertion to trigger. We can't change our API for
backward compatibility sake but same time we must not
preserve different behaviour between release and debug
builds since this brings inconsistency. Thus lets get
rid of assertion call and allow to call fiber_wakeup
in debug build as well.

Fixes #5843

NO_DOC=bug fix

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>