Add debug assert on duplicate freeClientAsync (#896)

When debug assert mode enabled, verify that we don't insert the same
client twice into server.clients_to_close.

Signed-off-by: naglera <anagler123@gmail.com>

src/networking.c

@@ -1775,6 +1775,7 @@ void freeClient(client *c) {
 void freeClientAsync(client *c) {
     if (c->flag.close_asap || c->flag.script) return;
     c->flag.close_asap = 1;
+    debugServerAssertWithInfo(c, NULL, listSearchKey(server.clients_to_close, c) == NULL);
     listAddNodeTail(server.clients_to_close, c);
 }