apparmor: avoid copying empty profile name

apparmor: avoid copying empty profile name

AppArmorGetSecurityProcessLabel copies the VM's profile name to the
label member of virSecurityLabel struct. If the profile is not loaded,
the name is set empty before calling virStrcpy to copy it. However,
virStrcpy will fail if src is empty (0 length), causing
AppArmorGetSecurityProcessLabel to needlessly fail. Simple operations
that report security driver information will subsequently fail

virsh dominfo test
Id:             248
Name:           test
...
Security model: apparmor
Security DOI:   0
error: internal error: error copying profile name

Avoid copying an empty profile name when the profile is not loaded.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>