vuls scan --debug cause `invalid memory address` error

[lv7777]

Environment

root@kaliProvisional:~/.ssh# uname -a
Linux kaliProvisional 4.6.0-kali1-amd64 #1 SMP Debian 4.6.4-1kali1 (2016-07-21) x86_64 GNU/Linux

Vuls

 commit 9dd2da8ed18ada09fc5b0442c2dd6e049ef0af05 

$ vuls -v

or
$ cd $GOPATH/src/github.com/future-architect/vuls 
$ git rev-parse --short HEAD 

## OS

kali linux

## Go
- Go version: go version go1.7.4 linux/amd64

# Current Output

`vuls configtest --debug` is path through.

the program is stopping after `Panic: runtime error: invalid memory address or nil pointer dereference on localhost`

```
zsh-common:
  Installed: 5.2-5
  Candidate: 5.3.1-3
  Version table:
     5.3.1-3 500
        500 http://http.kali.org/kali kali-rolling/main amd64 Packages
 *** 5.2-5 100
        100 /var/lib/dpkg/status

  stderr: 
  err: %!s(<nil>)
[Mar 22 08:56:25] DEBUG [localhost] Ensure changelog cache: localhost
[Mar 22 08:56:25] DEBUG [localhost] Panic: runtime error: invalid memory address or nil pointer dereference on localhost
^C
```


# Addition Details

I am tracking error point using "printf debug", and I revealed error point.

the memory access error is cause `ensureChangelogCache`

https://github.com/future-architect/vuls/blob/master/scan/debian.go#L276

`cached, found, err := cache.DB.GetMeta(current.Name)`

current.Name passed to `localhost`
I am debuged cache.DB.GetMeta,but this func is not called(when i settle `fmt.Printf` in cache.DB.GetMeta function,this is not called)




Can you also please fill in each of the remaining sections.

## Expected Behavior

plogram is not stop.

## Actual Behavior

plogram is stoping

## Steps to reproduce the behaviour
1. install vuls
2. write config.toml
```
[servers]

[servers.localhost]
host="127.0.0.1"
port="22"
user="root"
keyPath="/root/.ssh/id_rsa"
```
3. `vuls scan --debug`

[lv7777]

I am installed clean kali linux on vm, but the problem is occurred...

[knqyf263]

@lv7777 Thank you for the report!
I reproduced this issue and revealed the cause.
This is because cache.DB is null.

You must add kali to the following line
https://github.com/future-architect/vuls/blob/master/scan/serverapi.go#L425

Although this error will be resolved, the vulnerable packages will not be detected.
I tried to use apt-get changelog on Kali Linux, but I could not use it.

vuls uses changelog to detect the vulnerable packages.
So, it seems to be difficult to support Kali Linux right away...

[knqyf263]

Currently, Kali Linux is not supported.
Please watch the following issue.
#398

Thanks.