Drop use of partial List functions

Ghc-9.10 and later warn about use of partial functions like `List.head`.

Author: erikd

cardano-db-sync/src/Cardano/DbSync/Ledger/State.hs

@@ -705,10 +705,18 @@ listMemorySnapshots env = do
       case AS.toNewestFirst $ ledgerDbCheckpoints ldb of
         [] -> []
         [a] -> [a]
-        ls -> [List.head ls, List.last ls]
+        (h:ls) -> catMaybes [Just h, lastMaybe ls]
     notGenesis GenesisPoint = False
     notGenesis (BlockPoint _ _) = True
 
+    lastMaybe :: [a] -> Maybe a
+    lastMaybe xs =
+      case xs of
+        [] -> Nothing
+        [x] -> Just x
+        (_:ys) -> lastMaybe ys
+
+
 -- Get a list of the ledger state files order most recent
 listLedgerStateFilesOrdered :: LedgerStateDir -> IO [LedgerStateFile]
 listLedgerStateFilesOrdered dir = do