audioaggregator: fix input_buffer ownership
The way pad->priv->input_buffer reference was managed was pretty spurious:
- it was overridden without unrefing it, which could potentially lead to leaks.
- we were unreffing it while keeping the pointer around, which could potentially lead to use-after-free.
Clean it up by keeping the pointer as long as we actually have a strong reference on the buffer.
Fix a single buffer leak when shutting down the pipeline as the buffer returned from gst_aggregator_pad_drop_buffer() was never unreffed.