hack Block ctor&dtor to not do stuff when it won't produce output

this is a bloody hack but since the debug helper will be replaced with
something more reasonable for phonon five we really do not need a
durable approach here.

this solves excessive load issues when streaming via phonon as the
streamreader has loads of debug profiling instrumentation all over the
place, might want to remove those for some upcoming release perhaps as
the reader seems to be perfect as it is right now