test_add_conn: muted compiler warning

src/testcurl/test_add_conn.c

@@ -821,6 +821,8 @@ testExternalGet (void)
   if (! c_no_listen)
     c_d = curlEasyInitForTest ("http://127.0.0.1" EXPECTED_URI_FULL_PATH,
                                d_port, &cbc_d);
+  else
+    c_d = NULL; /* To mute compiler warning only */
 
   c_a = curlEasyInitForTest ("http://127.0.0.1" EXPECTED_URI_FULL_PATH,
                              a_port, &cbc_a);