Fix bug in burnSurplusAsFees where shortfall could be zero.

Fixes issue #3242.

This bug caused the following property to sporadically fail:

    "distributeSurplusDelta, when no change output is present, will burn
    surplus as excess fees"

Example failure seeds:

    --seed 862218635
    --seed 1926926873
    --seed 404630283
    --seed 1602363838
    --seed 1472406660
    --seed 31096920
    --seed 166643919

Failures were always of the form:

```hs
       expected: Right (TxFeeAndChange {fee = Coin 1, change = []})
        but got: Left (ErrMoreSurplusNeeded (Coin 0))
```

This indicates that it was possible for `burnSurplusAsFees` to fail with
an indicated shortfall of zero.

To fix this, we update `burnSurplusAsFees` to only fail if the computed
shortfall is non-zero.

We also adjust the following property to check that failure cases are
covered, and to verify that the shortfall is correct:

`prop_distributeSurplusDelta_coversCostIncreaseAndConservesSurplus`

lib/shelley/src/Cardano/Wallet/Shelley/Transaction.hs

@@ -1609,13 +1609,14 @@ burnSurplusAsFees
     -> Coin -- Surplus
     -> TxFeeAndChange ()
     -> Either ErrMoreSurplusNeeded (TxFeeAndChange ())
-burnSurplusAsFees feePolicy surplus (TxFeeAndChange fee0 ()) =
-    case costOfBurningSurplus `Coin.subtract` surplus of
-        Just shortfall -> Left $ ErrMoreSurplusNeeded shortfall
-        Nothing ->
-            Right $ TxFeeAndChange surplus ()
+burnSurplusAsFees feePolicy surplus (TxFeeAndChange fee0 ())
+    | shortfall > Coin 0 =
+        Left $ ErrMoreSurplusNeeded shortfall
+    | otherwise =
+        Right $ TxFeeAndChange surplus ()
   where
     costOfBurningSurplus = costOfIncreasingCoin feePolicy fee0 surplus
+    shortfall = costOfBurningSurplus `Coin.difference` surplus
 
 -- | Estimates the final size of a transaction based on its skeleton.
 --

lib/shelley/test/unit/Cardano/Wallet/Shelley/TransactionSpec.hs

@@ -270,7 +270,7 @@ import Crypto.Hash.Utils
 import Data.ByteString
     ( ByteString )
 import Data.Either
-    ( isRight )
+    ( isLeft, isRight )
 import Data.Function
     ( on, (&) )
 import Data.Functor.Identity
@@ -2409,10 +2409,18 @@ prop_distributeSurplusDelta_coversCostIncreaseAndConservesSurplus
     :: FeePolicy -> Coin -> Coin -> [Coin] -> Property
 prop_distributeSurplusDelta_coversCostIncreaseAndConservesSurplus
     feePolicy surplus fee0 change0 =
+    checkCoverage $
+    cover 2  (isLeft  mres) "Failure" $
+    cover 50 (isRight mres) "Success" $
+    report mres "Result" $
     counterexample (show mres) $ case mres of
-        Left _ ->
-            label "unable to distribute surplus" $
-                property (surplus < (maxCoinCost <> maxCoinCost))
+        Left (ErrMoreSurplusNeeded shortfall) ->
+            conjoin
+                [ property $ surplus < (maxCoinCost <> maxCoinCost)
+                , property $ shortfall > Coin 0
+                , costOfIncreasingCoin feePolicy fee0 surplus
+                    === shortfall <> surplus
+                ]
         Right (TxFeeAndChange feeDelta changeDeltas) -> do
             let feeRequirementIncrease = mconcat
                     [ costOfIncreasingCoin feePolicy fee0 feeDelta