If no matching device was found (cap == NULL) then no strdup() call
was made and *wwnn and *wwpn are untouched. Checking them for NULL
in this situation may result in reporting an false-positive OOM error
because *wwnn and *wwpn may be initialized to NULL by the caller.

Only check *wwnn and *wwpn for NULL if a matching device was found
(cap != NULL) and thus strdup() was called.

* src/conf/node_device_conf.c: only report an OOM error if there
  really is one