PATCH: blkparse: fix probem with empty trace files stopping the check for other files

PATCH: blkparse: fix probem with empty trace files stopping the check for other files

Fix problem where empty input files cause premature parsing of files.
Had a problem where CPU 0 opened a file with 0 traces, and that caused the
run to end before processing other trace files.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>