lavfi: use a new field for automatic buffer copy.

The code currently use cur_buf as the target of the copy,
but cur_buf can be cleared by the filter if it has given
the reference away or stored it elsewhere as soon as start_frame.

The code still relies on the fact that the reference is not
destroyed until end_frame. All filters currently follow that condition.
An av_assert1() is added to check it; it should at least cause
very visible errors in valgrind.