Set the request index to that of the current request (#8683)

* Set the request index to that of the current request

When setting up the chain of (two) requests to look up a block by hash, the second need to refer to the first. This fixes an issue where the back ref was set to the subsequent request, not the current one. When the requests are executed we loop through them in order and ensure the requests that should produce headers all match up. We do this by index so they better be right.

In other words: off by one.