tests: Don't call virNetServerClientClose without valid client

If @client hasn't been opened, then don't call virNetServerClientClose
since that'll cause certain failure.

Found by Coverity

Signed-off-by: John Ferlan <jferlan@redhat.com>

tests/virnetserverclienttest.c

@@ -152,7 +152,8 @@ static int testIdentity(const void *opaque ATTRIBUTE_UNUSED)
     ret = 0;
  cleanup:
     virObjectUnref(sock);
-    virNetServerClientClose(client);
+    if (!client)
+        virNetServerClientClose(client);
     virObjectUnref(client);
     virObjectUnref(ident);
     VIR_FORCE_CLOSE(sv[0]);