In a failing `Reader::VerifyEnd()`, fix annotating the status with the remaining length.
In a failing `Reader::VerifyEnd()`, fix annotating the status with the remaining
length.

Calling `Size()` after the `Reader` is failed will just return `absl::nullopt`,
so nothing was annotated. `Size()` must be called earlier.

(It was previously called after `Fail()` in order to leave a better `status()`
in the unlikely case where `SupportsSize()` is `true` but `Size()` fails.)

PiperOrigin-RevId: 436703976

google/riegeli,google/riegeli,google/riegeli,google/riegeli