LU-10772 utils: incorrect check in build_layout_from_yaml_node

LU-10772 utils: incorrect check in build_layout_from_yaml_node

In lustre/utils/lfs.c, build_layout_from_yaml_node(),
check and fail for NULL pointer 'string' before
dereferencing it.

Issue found in Static analysis.

Change-Id: I272432d3997f3bfae520ff3aa48d175309ccd386
Signed-off-by: Sonia Sharma <sonia.sharma@intel.com>
Reviewed-on: https://review.whamcloud.com/31537
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>