Fix off-by-one in hdrblobGet() making last entry unreachable (RhBug:1722921)
    
    hdrblobGet() introduced in commits acfde0d0e812e9f8e153ab6be8c9f2682bdd4763
    and 9821de18811db97238c34a564221e315f5f35b44 has an off-by-one thinko
    (perhaps the idea was to skip the first, region tag) which causes
    the last entry to be unreachable. In typical packages, that is
    RPMSIG_PAYLOADSIZE which is not used at all in this context so it doesn't
    matter, but in large packages use RPMSIG_LONGARCHIVESIZE which has a lower
    tag number and leaves either RPMSIGTAG_MD5 or RPMSIGTAG_GPG last,
    unreachable and thus unverifiable. Oops.
    
    This fixes the regression introduced in rpm 4.14, affecting verification
    of large packages (ie having RPMSIG_LONGARCHIVESIZE)