src: fix resource leak introduced in d4439a6b8

@tmp that was copied just above is leaked on plain return.
The issue is found by Coverity.

Patch that inroduced a leak:
d4439a6b8 : src: adopt to VIR_DRV_SUPPORTS_FEATURE return -1

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

src/libvirt-domain.c

@@ -3022,7 +3022,7 @@ virDomainMigrateVersion3Full(virDomainPtr domain,
     ret = VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
                                    VIR_DRV_FEATURE_MIGRATE_CHANGE_PROTECTION);
     if (ret < 0)
-        return NULL;
+        goto done;
     if (ret)
         protection = VIR_MIGRATE_CHANGE_PROTECTION;