fix: nil pointer when no match for any OS

[kotakanbe]

What did you implement:

fix: nil pointer if no match for any OS

# mv /etc/debian_version /etc/debian_version.bak

# ./vuls scan --debug
[Feb 23 00:15:48]  INFO [localhost] vuls-v0.19.4-build-20220223_001011_7d8a24e
[Feb 23 00:15:48]  INFO [localhost] Start scanning
[Feb 23 00:15:48]  INFO [localhost] config: /go/src/github.com/future-architect/vuls.clean/config.toml
...
[Feb 23 00:15:48] DEBUG [localhost] Panic: runtime error: invalid memory address or nil pointer dereference on localhost

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Confirmed that no nil pointer occurs.

Checklist:

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES