Initialize the allocation counter in GHCi to 0 (Fixes #16012)

According to the documentation for the function `getAllocationCounter` in
[System.Mem](http://hackage.haskell.org/package/base-4.14.0.0/docs/System-Mem.html)
initialize the allocationCounter also in GHCi to 0.

libraries/ghci/GHCi/Run.hs

@@ -250,10 +250,10 @@ redirectInterrupts target wait = do
 
 measureAlloc :: IO (EvalResult a) -> IO (EvalStatus a)
 measureAlloc io = do
-  setAllocationCounter maxBound
+  setAllocationCounter 0                                 -- #16012
   a <- io
   ctr <- getAllocationCounter
-  let allocs = fromIntegral (maxBound::Int64) - fromIntegral ctr
+  let allocs = negate $ fromIntegral ctr
   return (EvalComplete allocs a)
 
 -- Exceptions can't be marshaled because they're dynamically typed, so

testsuite/tests/ghci/should_run/T16012.script

+n <- System.Mem.getAllocationCounter
+(n < 0 && n >= -160000)

testsuite/tests/ghci/should_run/T16012.stdout

+True

testsuite/tests/ghci/should_run/all.T

@@ -62,6 +62,7 @@ test('T15633b',
       ],
      ghci_script, ['T15633b.script'])
 
+test('T16012', just_ghci, ghci_script, ['T16012.script'])
 test('T16096', just_ghci, ghci_script, ['T16096.script'])
 test('T507', just_ghci, ghci_script, ['T507.script'])
 test('T18027', just_ghci, ghci_script, ['T18027.script'])