libxl: fix libxlDriverConfigDispose for partially constructed object

libxlDriverConfigNew() use libxlDriverConfigDispose() for cleanup in
case of errors. Do not call libxlLoggerFree() on not allocated logger
(NULL).
Signed-off-by: NMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: NJim Fehlig <jfehlig@suse.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/libxl/libxl_conf.c

@@ -80,7 +80,8 @@ libxlDriverConfigDispose(void *obj)
 
     virObjectUnref(cfg->caps);
     libxl_ctx_free(cfg->ctx);
-    libxlLoggerFree(cfg->logger);
+    if (cfg->logger)
+        libxlLoggerFree(cfg->logger);
 
     VIR_FREE(cfg->configDir);
     VIR_FREE(cfg->autostartDir);