fdread: Fix logic bug when reading by sector

When reading by sector (because reading a whole track failed), we can
accidentally fall into the "should not happen" path, which both
(a) emits a spurious error message and (b) fouls up our position
accounting going forward. Ensure we do not inappropriately fall into
that path.

Avoid obscuring the "short after" message in cases where it happens.

Signed-off-by: Matt Jacobson <mhjacobson@me.com>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1897