dlm: fix possible call to kfree() for non-initialized pointer

Technically dlm_config_nodes() could return error and keep nodes
uninitialized. After that on the fail path of we'll call kfree()
for that uninitialized value.

The patch is simple - we should just initialize nodes with NULL.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David Teigland <teigland@redhat.com>