LU-6612 utils: strengthen llog_reader vs wrong format/header

LU-6612 utils: strengthen llog_reader vs wrong format/header

The following snippet shows that llog_reader can be puzzled due to
an invalid 0 for the number of records when parsing an expected
LLOG file header :
root# dd if=/dev/zero bs=4096 count=1 of=/tmp/zeroes
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.000263962 s, 15.5 MB/s
root# llog_reader /tmp/zeroes
Memory Alloc for recs_buf error.
Could not pack buffer; rc=-12

Test-Parameters: trivial testlist=sanity,sanity-hsm
Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I12be79e6c6a5da384a5fd81878a76a7ea8aa5834
Reviewed-on: https://review.whamcloud.com/15654
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>