Fix for input readers occasionally dead-locking.

Bernhard and I tracked it down we believe: the thread queue could
deadlock in certain cases. As a fix we tuned the heuristic for telling
if a queue might have input to occasionaly err on the safe side by
flagging "yes", so that processing will proceed.

It's a bit unfortunate to apply this fix last minute before the
release as it could potentially impact performance if the heuristic
fails to often. We believe the chosen parmaterization should be fine ...