aggregator: Reset EOS flag after receiving a stream-start event

aggregator: Reset EOS flag after receiving a stream-start event

And also don't assert that there are no buffers queued up when handling
an EOS event. The pad's streaming thread might've already received a new
stream-start event and queued up a buffer in the meantime.

This still leaves a race condition where the srcpad task sees all pads
in EOS state and finishes the stream, while shortly afterwards a pad
might receive a stream-start event again, but this doesn't seem to be
solveable with the current aggregator design.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2769>