empty strings should be allowed with %b

[nicolasff]

Empty strings cause the command builder to create an invalid request.

    /* Set a key using binary safe API */
    reply = redisCommand(c,"SET %b %b", "foo", 3, "", 0);
    printf("SET (binary API): %s\n", reply->str);
    freeReplyObject(reply);

Responds with:

SET (binary API): ERR wrong number of arguments for 'set' command

The following command is generated and sent:

*2
$3
SET
$3
foo

This issue was discovered whilst porting phpredis to a hiredis backend; a unit test failed.

[pietern]

Thanks for the report. This is now fixed on master.