Start http1 header read timeout even when 0 bytes are read

[jeromegn]

As mentioned in https://github.com/hyperium/hyper/pull/2675#issuecomment-1666309018, the timeout didn't apply if no bytes were written yet.

This moves the start of the timeout's polling before the check for an empty buffer.

[seanmonstar]

FWIW, this sounds very similar to this other PR, which I paused because of the follow comment: https://github.com/hyperium/hyper/pull/3185#issuecomment-1489170787.