LU-15130 nrs: null pointer dereference in nrs_tbf_id_parse

LU-15130 nrs: null pointer dereference in nrs_tbf_id_parse

cfs_gettok() set next->ls_str to NULL if no delimiter is found but
it does not update next->ls_len to 0.
We have to check if next->ls_str is null inside nrs_tbf_id_parse()
to verify if the tbf expression is valid.

* Reproducer *
lctl set_param  mds.MDS.mdt.nrs_tbf_rule="start tbf_name gid{500}
rate=100"

This patch fix cfs_gettok() to update "next->ls_len = 0;" if no
delimiter is found.

Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: Iaa4eb5085262cee547ea3a944ddb94c6df1f8aa3
Reviewed-on: https://review.whamcloud.com/45291
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>