tests: Ensure queue size is always positive

Queue.empty was being called with negative sizes

tests/Data/BlockQueueSpec.hs

@@ -10,9 +10,6 @@ import           Test.QuickCheck
 import           Control.Monad.ST
 import qualified Data.List as List (delete)
 
-import           Data.Vector (Vector)
-import qualified Data.Vector as V
-
 import           Data.BlockQueue
 import           Data.RefinablePartition (Block(..))
 
@@ -24,10 +21,10 @@ spec = do
 emptySpec :: Spec
 emptySpec = describe "empty" $ do
   it "creates an empty queue" $
-    property $ \sizes -> runST $ empty sizes >>= null
+    property $ \size -> size >= 0 ==> runST $ empty size >>= null
 
   it "creates a queue where no block is present" $
-    property $ \size -> runST $ do
+    property $ \size -> size >= 0 ==> runST $ do
       q <- empty size
       all not <$> mapM (\b -> Block b `elem` q) [0..size-1]
 
@@ -55,7 +52,3 @@ enqueueSpec = describe "enqueue" $ do
             Just 0 <- dequeue q
             null q
       in  runST enqtwice `shouldBe` True
-
-instance Arbitrary a => Arbitrary (Vector a) where
-  arbitrary = V.fromList <$> arbitrary
-  shrink = fmap V.fromList . shrink . V.toList