LU-10772 utils: incorrect NULL check in free_node()

LU-10772 utils: incorrect NULL check in free_node()

In lnet/utils/lnetconfig/cyaml.c, for free_node()
check first for NULL pointer before dereferencing it.

Issue found in Static analysis

Change-Id: I6298f0f09175b6fd210db5717d44d050b1cb9d8d
Signed-off-by: Sonia Sharma <sonia.sharma@intel.com>
Reviewed-on: https://review.whamcloud.com/31551
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>