LU-7800 llog: check if next llog exists

LU-7800 llog: check if next llog exists

Because next llog creation will only be checked
and created in declare phase, and it does not
serialize the catllog accessing in the whole
declare_add and add process, so if there are
mulitple threads access the catlog at the same
time, and if the llog creation did not succeeds,
then next_log (in catllog) might be NULL, and
it will cause panic in llog_cat_current_log().

Signed-off-by: Di Wang <di.wang@intel.com>
Change-Id: I2343023c1f3109c077c98d78d3669377d95ed42f
Reviewed-on: http://review.whamcloud.com/18542
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>