Fix bug by increasing size by 1

It's not possible to reproduce the second bug because `mod` operator
in Haskell is implementeed correctly, not like `rem` in C or Erlang.

> (-1) `mod` 2 == 1

quickcheck-dynamic/test/Spec/CircularBuffer/Buffer.hs

@@ -36,10 +36,12 @@ instance Eq Buffer where
 -- | See 'New'.
 newBuffer :: Int -> IO Buffer
 newBuffer n =
-  Buffer n
+  Buffer size
     <$> newIORef 0
     <*> newIORef 0
-    <*> V.new n
+    <*> V.new size
+  where
+    size = n + 1
 
 -- | See 'Put'.
 putBuffer :: Int -> Buffer -> IO ()
@@ -61,4 +63,4 @@ lenBuffer :: Buffer -> IO Int
 lenBuffer Buffer{inp, outp, size} = do
   i <- readIORef inp
   j <- readIORef outp
-  return $ (i - j) `mod` size
+  pure $ (i - j) `mod` size

quickcheck-dynamic/test/Spec/CircularBuffer/Model.hs

@@ -13,8 +13,8 @@ import Control.Monad.Trans (lift)
 import Data.Maybe (fromJust)
 import GHC.Generics (Generic)
 import Spec.CircularBuffer.Buffer (Buffer, getBuffer, lenBuffer, newBuffer, putBuffer)
-import Test.QuickCheck (Arbitrary (..), Property, counterexample, getPositive, oneof)
-import Test.QuickCheck.DynamicLogic (DL, DynLogicModel (..), action, anyActions_, assert, assertModel, forAllDL, getModelStateDL, monitorDL)
+import Test.QuickCheck (Arbitrary (..), Property, getPositive, oneof)
+import Test.QuickCheck.DynamicLogic (DL, DynLogicModel (..), forAllDL)
 import Test.QuickCheck.Extras (runPropertyStateT)
 import Test.QuickCheck.Monadic (monadicIO)
 import Test.QuickCheck.Monadic qualified as QC
@@ -113,5 +113,4 @@ tests :: TestTree
 tests =
   testGroup
     "Circular Buffer"
-    [ testProperty "implementation respects its model" prop_CircularBuffer
-    ]
+    [testProperty "implementation respects its model" prop_CircularBuffer]