aggregator: Release pads' peeked buffer when removing the pad or finalizing it

aggregator: Release pads' peeked buffer when removing the pad or finalizing it

The peeked buffer was always reset after calling ::aggregate() but under
no other circumstances. If a pad was removed after peeking and before
::aggregate() returned then the peeked buffer would be leaked.

This can easily happen if pads are removed from the aggregator from a
pad probe downstream of the source pad but still in the source pad's
streaming thread.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/784>