LU-12420 utils: llog_reader handles uninitialized llog properly

LU-12420 utils: llog_reader handles uninitialized llog properly

When reading an empty LLOG, llog_reader would crash because
of record number of zero. E.g. "llog_reader CONFIGS/nodemap" on
a MGS without nodemap configuration would cause failure of:

llog_reader: Error allocating -16 bytes for recs_buf: Cannot allocate memory (12)
llog_reader: Could not pack buffer.: Cannot allocate memory (12)

After apply this patch, llog_reader will print following message
and quit if the LLOG is unintialized:

uninitialized llog: zero record number

Change-Id: I87246672e9fc992c99126134236c2e8d304df74b
Signed-off-by: Li Xi <lixi@ddn.com>
Reviewed-on: https://review.whamcloud.com/35177
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>