core,eth/tracers: don't panic in OnTxEnd if receipt is nil

[lightclient]

There are two issues currently with the tracer's OnTxEnd invocation in state_processor

1. Because it is deferred and uses named return values, the receipt is not yet set in the first return after ApplyTransactionWithEVM. OnTxEnd expects the receipt to be non-nil, so this causes a panic.
2. The error from result is never set into the named return value err. So the error reporting for OnTxEnd isn't really correct.

This PR fixes both of these issues.

cc @s1na

[s1na]

1. Hm I thought it is expected that in case of error the receipt will be null. It can be that the tracers are not handling this well?
2. Yep so here the OnTxEnd captures tx-level errors like insufficient intrinsic gas etc. The "vm errors" will be caught by lower level hooks like OnExit or OnFault.

[lightclient]

Okay I see, I updated the PR to just fix 1). Thanks

[s1na]

I'm surprised you've seen a panic here. As far as I can see in state processor either receipt is nil and error is non-nil (in which case struct logger returns) or there is a receipt.

[lightclient]

Ah you're right. The issue issue is actually that err is redeclared. Sorry it actually wasn't a problem in the state_processor, it's in the test runner.

[lightclient]

Closing this for now as my change doesn't address the issue and the current code in state_processor works. The issue is there are some call sites for OnTxEnd which do not pass a receipt, e.g.

go-ethereum/tests/state_test_util.go

Line 302 in 2d9d423

evm.Config.Tracer.OnTxEnd(nil, err)