Fix ref-counting when CompositeByteBuf is used with retainedSlice() (#8497)

Motivation:

ByteBuf.retainedSlice() and similar methods produce sliced buffers with
an independent refcount to the buffer that they wrap.


One of the optimizations in 10539f4d was
to use the ref to the unwrapped buffer object for added slices, but this
did not take into account the above special case when later releasing.

Thanks to @rkapsi for discovering this via #8495.

Modifications:

Since a reference to the slice is still kept in the Component class,
just changed Component.freeIfNecessary() to release the slice in
preference to the unwrapped buf.

Also added a unit test which reproduces the bug.

Result:

Fixes #8495