vaapioverlay: ensure sinkpad has current buffer

Use the gst_video_aggregator_pad_has_current_buffer API
to check if the current sinkpad has a queued buffer before
attempting to obtain a input buffer from the base plugin.
If the sinkpad does not have a current buffer, then it is
either not producing them yet (e.g. current time < sinkpad
start time) or it has reached EOS.

Previously, we only handled EOS case.

Example:

 gst-launch-1.0 videotestsrc num-buffers=100 \
  ! vaapipostproc ! vaapioverlay name=overlay \
  ! vaapisink videotestsrc timestamp-offset=1000000000 \
  num-buffers=100 ! video/x-raw,width=160,height=120 \
  ! overlay.

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