Fix Sentinel memory leak (hiredis bug)

This fixes issue #2535, that was actually an hiredis library bug (I
submitted an issue and fix to the redis/hiredis repo as well).

When an asynchronous hiredis connection subscribes to a Pub/Sub channel
and gets an error, and in other related conditions, the function
redisProcessCallbacks() enters a code path where the link is
disconnected, however the function returns before freeing the allocated
reply object. This causes a memory leak. The memory leak was trivial to
trigger in Redis Sentinel, which uses hiredis, every time we tried to
subscribe to an instance that required a password, in case the Sentinel
was configured either with the wrong password or without password at
all. In this case, the -AUTH error caused the leaking code path to be
executed.

It was verified with Valgrind that after this change the leak no longer
happens in Sentinel with a misconfigured authentication password.

deps/hiredis/async.c

@@ -443,6 +443,7 @@ void redisProcessCallbacks(redisAsyncContext *ac) {
             if (((redisReply*)reply)->type == REDIS_REPLY_ERROR) {
                 c->err = REDIS_ERR_OTHER;
                 snprintf(c->errstr,sizeof(c->errstr),"%s",((redisReply*)reply)->str);
+                c->reader->fn->freeObject(reply);
                 __redisAsyncDisconnect(ac);
                 return;
             }