add check for sector_shift == 0, since that's technically invalid, but note what one source says that might be true.
add check for sector_shift == 0, since that's technically invalid, but
note what one source says that might be true.

joncampbell123/doslib,joncampbell123/doslib,joncampbell123/doslib,joncampbell123/doslib,joncampbell123/doslib,joncampbell123/doslib