witness gas consumption costs aren't always verfied

[gballet]

Got this while debugging the shadow fork:

panic: gas pool pushed above uint64

goroutine 4313 [running]:
github.com/ethereum/go-ethereum/core.(*GasPool).AddGas(...)
        github.com/ethereum/go-ethereum/core/gaspool.go:31
github.com/ethereum/go-ethereum/core.(*StateTransition).refundGas(0xc0247db208, 0x1778f9993fd00a15?)
        github.com/ethereum/go-ethereum/core/state_transition.go:502 +0x188
github.com/ethereum/go-ethereum/core.(*StateTransition).TransitionDb(0xc0247db208)
        github.com/ethereum/go-ethereum/core/state_transition.go:464 +0xff0
github.com/ethereum/go-ethereum/core.ApplyMessage(0x5aad1f?, 0xc0039712c0?, 0x1aa3f20?)
        github.com/ethereum/go-ethereum/core/state_transition.go:183 +0x5d
github.com/ethereum/go-ethereum/core.applyTransaction(0xc0274a2000, 0xc0005921a0, 0xc02ad9e0f8?, 0xc0007e83c0, 0xc02c8d2060, {0xaa, 0x65, 0x79, 0x9d, 0x27, ...}, ...)
        github.com/ethereum/go-ethereum/core/state_processor.go:120 +0x585
github.com/ethereum/go-ethereum/core.ApplyTransaction(0x6f6637?, {0x20afab0, 0xc000221000}, 0xc003dd9290?, 0xc003967b08?, 0x459889?, 0xc003ba5900, 0xc031f85680, 0x7f2bf1b38f18?, {{0x0, ...}, ...})
        github.com/ethereum/go-ethereum/core/state_processor.go:173 +0x2b9
github.com/ethereum/go-ethereum/miner.(*worker).commitTransaction(0xe4e3c6d5923d9969?, 0xc0139f3a00, 0xc032b13db0)
        github.com/ethereum/go-ethereum/miner/worker.go:736 +0x1d8
github.com/ethereum/go-ethereum/miner.(*worker).commitTransactions(0xc000132b40, 0xc0139f3a00, 0xc034346cc0, 0x0)
        github.com/ethereum/go-ethereum/miner/worker.go:794 +0x4d8
github.com/ethereum/go-ethereum/miner.(*worker).mainLoop(0xc000132b40)
        github.com/ethereum/go-ethereum/miner/worker.go:550 +0x96a
created by github.com/ethereum/go-ethereum/miner.newWorker
        github.com/ethereum/go-ethereum/miner/worker.go:282 +0x84a

which corresponds to a negative consumed gas. This doesn't appear when running the shadow fork with 0 witness gas costs, so that indicates that somewhere, the witness gas consumption isn't checked.