fix BuildBlocksValidator: use bytes buffer pool correctly

[zhiqiangxu]

The original implementation is essentially the same as passing nil as dst and thus is a waste of memory.

And there's no need to update res unless data is larger.

This PR also simplifies expressions like:

payload.BlobGasUsed == nil || (payload.BlobGasUsed != nil && *payload.BlobGasUsed != 0)

to:

payload.BlobGasUsed == nil || *payload.BlobGasUsed != 0.

[protolambda]

Nice catch. It looks like the underlying snappy.Decode function uses len, not cap, to determine if the destination-slice can grow to the necessary capacity. So passing in a [:0] indeed doesn't help. The len(data) > len(*res) would be better to use cap though, since we care about the available size, not the utilized size.

Also, there is a similar issue in BuildMsgIdFn. I'll update it to fix that also.