rgw: fix error_repo segfault in data sync

rgw: fix error_repo segfault in data sync

RGWDataSyncShardCR will only allocate an error_repo if it's doing
incremental sync, so RGWDataSyncSingleEntryCR needs to guard against a
null error_repo

also, RGWDataSyncShardCR::stop_spawned_services() was dropping the last
reference to the error_repo before calling drain_all(), which meant that
RGWDataSyncSingleEntryCR could still be holding a pointer. now uses a
boost::intrusive_ptr in RGWDataSyncSingleEntryCR to account for its
reference

Fixes: http://tracker.ceph.com/issues/16603
Signed-off-by: Casey Bodley <cbodley@redhat.com>