Issue title: panic: runtime error: slice bounds out of range with gor listen
 Issue body: I'm trying new gor versions (0.8.0 and 0.8.1). With gor listen and above 70 requests per second, after 5min aprox I get this error:

panic: runtime error: slice bounds out of range.

I uploaded the full error trace here: https://gist.github.com/5973428f68a7016b8618

 Comments: 
 Comment 0: are you running the replay and listener on the same server? could you give an example of command-line arguments you are using?

 Comment 1: no, in different servers.

this is the command line call:

`sudo /usr/sbin/gor --input-raw :5001 --output-tcp myreplayserver-example.net:28020`

I tried with GOMAXPROCS too:

`sudo GOMAXPROCS=10 /usr/sbin/gor --input-raw :5001 --output-tcp myreplayserver-example.net:28020`

 Comment 2: pls try this one: https://github.com/buger/gor/releases/tag/v0.8.2

 Comment 3: Sorry but it doesn't work for me. This version doesn't replay traffic. With the verbose option enabled, it doesn't log anything.

 Comment 4: I'm seeing the same issue and upgraded to 0.8.2. As @areina already said, replay does not work anymore.

My command line was:

```
./gor --input-raw :80 --output-http "http://dev3.xyz.io:1142"
```

 Comment 5: I'm seeing the same issue with v0.8.2

`sudo GOMAXPROCS=24 gor --input-raw :9098 --output-http "http://chellishpm001:9097"`

 Comment 6: I also tried with separate instances of the listener and replay and I see the same problem.

`sudo GOMAXPROCS=24 gor --input-raw :9098 --output-tcp chellishpm001:28020`

`gor --input-tcp :28020 --output-http "http://chellishpm001:9097"`

{'source': 'GithubIssues', 'repo_name_with_owner': 'buger/goreplay', 'issue_id': 29767653, 'repo_stars': 16286.0}