... which clears the drawable's buffer, performing any necessary
cleanup, without setting a new buffer.  While the drawable has no
buffer, it can only be used in a very limited way, in particular,
it may be destroyed, and it may be assigned a new buffer.

This is used by the next commit to implement
gimp_drawable_steal_buffer(), which transfers a buffer from one
drawable to another in a safe manner, leaving the source drawable
empty.