Fix large string copies

[jochem-brouwer]

When implementing Homestead in this PR I ran into an issue when trying to copy SHA3 large buffers which is caused by assertIsBuffer. This copies Buffers with sizes which node cannot handle even if there are no errors:

Error: Cannot create a string longer than 0x3fffffe7 characters
    at Buffer.toString (buffer.js:777:17)
    at Object.exports.assertIsBuffer (/Volumes/Ethereum/ethereumjs-vm/node_modules/ethereumjs-util/src/helpers.ts:19:15)
    at Object.exports.keccak (/Volumes/Ethereum/ethereumjs-vm/node_modules/ethereumjs-util/src/hash.ts:14:3)
    at Object.exports.keccak256 (/Volumes/Ethereum/ethereumjs-vm/node_modules/ethereumjs-util/src/hash.ts:39:10)

When you try to copy a Buffer into a string which is too large, it throws. Besides that, it does not make sense to create these messages if there are no errors (this is unnecessary copying).

[jochem-brouwer]

Check won't pass because coverage doesn't upload?

[alcuadrado]

LGTM. Makes sense not to create the strings if they aren't needed.

I don't know what's going on with the CI. I tried re-running it and it failed in another way.

[jochem-brouwer]

@evertonfraga could you take a look why Upload to Coveralls is not working in the CI? 😄

[evertonfraga]

Sure, sir!

[github-actions]

Coverage remained the same at 99.711% when pulling fbaf1b8 on fix-helper-memcopy into c6e8f3e on master.

[evertonfraga]

Done and fixed. Feel free to merge!

[evertonfraga]

The main reason was a command trying to fetch a SHA reference from git, when it didn't exist. Checkout version 2 fixes that.

##[error]Error: Command failed: git cat-file -p 3130d061cb32a5063fafc16788945bd0cce0d55b

[jochem-brouwer]

Thanks a lot @evertonfraga 😄