Make sure to close the reader created by flush_fragments if an exception occurs before it's moved to `populate_views`. Note that it is also ok to close the reader _after_ it has been moved, in case populate_views itself throws after closing the reader that was moved it. For conveience flat_mutation_reader::close supports close-after-move. Fixes #9479 Signed-off-by: Benny Halevy <bhalevy@scylladb.com> Message-Id: <20211024164138.1100304-1-bhalevy@scylladb.com>