storage: skip selinux cleanup when fd not available

storage: skip selinux cleanup when fd not available

When attempting to backport gluster pools to an older versoin
where there is no VIR_STRDUP, I got a crash from calling
strdup(,NULL). Rather than relying on the current else branch
safely doing nothing when there is no fd, it is easier to just
skip it. While at it, there's no need to explicitly set
perms.label to NULL after a VIR_FREE().

* src/storage/storage_backend.c
(virStorageBackendUpdateVolTargetInfoFD): Minor optimization.

Signed-off-by: Eric Blake <eblake@redhat.com>