LU-2059 mgs: fix a bug in mgs_llog_open

LU-2059 mgs: fix a bug in mgs_llog_open

- don't calculate len for NULL ptr
- check the logname is PARAMS_FILENAME inside 'if (ptr == NULL ...)
clause so all cases with NULL ptr will end there.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: Ib87ecb3245833fc8f9d069898a1b80c44bbfd9f0
Reviewed-on: http://review.whamcloud.com/10589
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>