Only use sample labels for blank files
Only use sample labels for blank files

The unwanted behavior of also using sample labels whenever there was a
problem reading the file header was introduced in:

    2013-02-25 Fix: Missing log file header causes error

The sample labels should not be used if they are missing from the file
header or if the header is missing or corrupt because the user could
have intentionally deleted them.

jazzhead/log-page,jazzhead/log-page