[axe] runtime error avec -featureStatEvents

Avec Canopsis 3.31.0, quand l'option -featureStatEvents est activée, axe plante assez rapidement avec la trace suivante :

goroutine 167 [running]:
runtime/debug.Stack(0xc00006dbc0, 0xc000084ea0, 0x5d)
  /usr/local/go/src/runtime/debug/stack.go:24 +0x9d
runtime/debug.PrintStack()
  /usr/local/go/src/runtime/debug/stack.go:16 +0x22
git.canopsis.net/canopsis/go-engines/lib/canopsis.(*DefaultEngine).RecoverWorker(0xc0003089c0)
  git.canopsis.net/canopsis/go-engines/lib/canopsis/engine.go:256 +0xd6
panic(0xbe2d00, 0x113ac70)
  /usr/local/go/src/runtime/panic.go:522 +0x1b5
main.(*EngineAxe).WorkerProcess(0xc0003089c0, 0xd4c0c0, 0xc00027aa20, 0x0, 0xc00021f060, 0x10, 0x0, 0x0, 0x2, 0x0, ...)
  git.canopsis.net/canopsis/go-engines/cmd/engine-axe/engine.go:251 +0xc96
git.canopsis.net/canopsis/go-engines/lib/canopsis.routineWorker(0xd56ea0, 0xc0003089c0, 0xc00023f080)
  git.canopsis.net/canopsis/go-engines/lib/canopsis/engine.go:290 +0x24d
created by git.canopsis.net/canopsis/go-engines/lib/canopsis.StartEngine
  git.canopsis.net/canopsis/go-engines/lib/canopsis/engine.go:235 +0x20c
2019-10-31T08:21:09Z ERR git.canopsis.net/canopsis/go-engines/lib/canopsis/engine.go:255 > worker recovered from panic: runtime error: invalid memory address or nil pointer dereference
2019-10-31T08:21:09Z INF git.canopsis.net/canopsis/go-engines/lib/canopsis/engine.go:169 > engine stop called, waiting for processes to finish.