volserver: Do not return ENOMEM on AIX from XVolListPartitions

volserver: Do not return ENOMEM on AIX from XVolListPartitions

When calling "vos partinfo" or "vos listpart" towards a server
 running AIX with no partitions attached, it would return a
ENOMEM, because unlike on linux, malloc(0) returns NULL on AIX.
Thus, just don't do any malloc, when we have no partitions anyway.

Change-Id: Id1900e2ab11850ada8b2e91667288576d408014b
Reviewed-on: http://gerrit.openafs.org/2912
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>