Move initial iterateVm checks to after step hook

[gnidan]

To ensure those conditions still generate a valid step for runStepHook handler.

The initial checks otherwise prevent certain situations from generating steps (e.g. for trace output)

[coveralls]

Coverage remained the same at 85.462% when pulling c37298e on gnidan:patch-1 into 688aec6 on ethereumjs:master.

[gnidan]

Thanks @axic!

[axic]

@cdetrio does tracing outputs the last opcode even if it fails?

[gnidan]

@axic trace steps are structured so that the program counter refers to the opcode about to happen. so in failing transactions, I would think you'd want to see the last opcode? cause that's the one that "broke"? (Although I'm not sure how much of what I just said is debug_traceTransaction specific, vs. ethereumjs-vm behavior)

[axic]

It should match what debug_traceTransaction needs

[axic]

@cdetrio @jwasinger merging this now

[axic]

The readme also stated:

opcode - the next opcode to be ran

So this change should be close to what the readme is saying.