ReplicatedPG::start_flush: fix clone deletion case

dsnapc.snaps will be non-empty most of the time if there
have been snaps before prev_snapc.  What we really want to
know is whether there are any snaps between oi.snaps.back()
and prev_snapc.

Fixes: 8334
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>