Prevent crash of libvirtd when attaching to existing qemu process

With security_driver set to "none" in /etc/libvirt/qemu.conf,
libvirtd would crash when attempted to attach to an existing
qemu process.  Only copy the security model if it actually exists.

src/qemu/qemu_process.c

@@ -3609,7 +3609,8 @@ int qemuProcessAttach(virConnectPtr conn ATTRIBUTE_UNUSED,
     if (virSecurityManagerGetProcessLabel(driver->securityManager,
                                           vm, seclabel) < 0)
         goto cleanup;
-    if (!(vm->def->seclabel.model = strdup(driver->caps->host.secModel.model)))
+    if (driver->caps->host.secModel.model &&
+        !(vm->def->seclabel.model = strdup(driver->caps->host.secModel.model)))
         goto no_memory;
     if (!(vm->def->seclabel.label = strdup(seclabel->label)))
         goto no_memory;