protect the cache from DNS responses that lack an Answer, add .gitign…

[mikerott]

Two changes in this PR:

1. protect the DNS cache from entries that lack an Answer (explanation below)
2. add .gitignore to ignore vendor/ folder

In testing Nuclei as an embedded library in an AWS lambda (even worse, in a lambda being run in localstack container in docker-compose), with a bit of fmt.Print* to print the value of b here:

b, _ := data.Marshal()
err = d.hm.Set(hostname, b)

I observed that when passing both https://something and http://something into the executor with Nuclei configured for BulkSize greater than 1, the value of b occasionally lacked an Answer, which would populate the cache with a bad entry.

I honestly don't know if it's due to odd behavior of docker-compose internal DNS, or a go bug or what, but the change in this PR adds some protection for the cache.

Related to this discussion: projectdiscovery/nuclei#2277

Detail:

I added code like:

if b != nil {
    fmt.Printf("MIKE: b is %v\n", b)  // not string(b) because for some reason the lambda panicked
}

Then inspected the output to observe there was no Answer in the b passed to d.hm.Set in some cases. I could not reliably reproduce the problem, but it was frequent enough to warrant the investigation and this PR.