fix crash in drop scan when there are no associated files

maximum is partial, so need to ensure the list is not empty.

This lack of associated files would most likely be a problem -- and fsck
fixes it. It could also occur legitimately due to a race.

Assistant/Drop.hs

@@ -45,7 +45,11 @@ handleDropsFrom :: [UUID] -> [Remote] -> Reason -> Bool -> Key -> AssociatedFile
 handleDropsFrom _ _ _ _ _ Nothing _ = noop
 handleDropsFrom locs rs reason fromhere key (Just afile) knownpresentremote = do
 	fs <- liftAnnex $ ifM isDirect
-		( associatedFilesRelative key
+		( do
+			l <- associatedFilesRelative key
+			if null l
+				then return [afile]
+				else return l
 		, return [afile]
 		)
 	n <- getcopies fs