avconv: restructure sending EOF to filters

avconv: restructure sending EOF to filters

Be more careful when an input stream encounters EOF when its filtergraph
has not been configured yet. The current code would immediately mark the
corresponding output streams as finished, while there may still be
buffered frames waiting for frames to appear on other filtergraph
inputs.

This should fix the random FATE failures for complex filtergraph tests
after a3a0230a9870b9018dc7415ae5872784d524cfe5