getEvent can't deserialize event serialized by putEvent

ghc-events currently fails to deserialize some events that it serializes. To reproduce, first generate a minimal eventlog file from the program `main = return ()` (tried GHC 8.4), then run

```haskell
import Data.Binary.Get
import Data.Binary.Put
import qualified Data.ByteString as BS
import qualified Data.ByteString.Lazy as LBS
import GHC.RTS.Events
import GHC.RTS.Events.Binary
import GHC.RTS.Events.Incremental

main = do
bs <- LBS.readFile "empty.eventlog"
let header = runGet getHeader bs
let parsers = mkEventParsers header

let event = Event
{ evTime = 415731793
, evSpec = GCStatsGHC
{ heapCapset = 0
, gen = 1
, copied = 305104
, slop = 250088
, frag = 3088384
, parNThreads = 12
, parMaxCopied = 4246
, parTotCopied = 305104
}
, evCap = Nothing
}

print (runGet (getEvent parsers) (runPut (putEvent event)))
```

Output:

```
bug: Data.Binary.Get.runGet at position 60: not enough bytes
CallStack (from HasCallStack):
error, called at libraries/binary/src/Data/Binary/Get.hs:351:5 in binary-0.8.5.1:Data.Binary.Get
```

Start with GHC.RTS.Events.Binary's getEvent and putEvent, using the provided reproduction with mkEventParsers and the minimal eventlog generated from `main = return ()`. Confirm the runGet/runPut round trip failure and trace the serialized GCStatsGHC fields; done means events produced by putEvent deserialize successfully with getEvent.

Written by the indexing model from the issue text.