DRY out repeated code for detecting malformed scan targets

[dav3r]

💡 Summary

DRY out repeated code for detecting malformed scan targets introduced in #17

Motivation and context

As suggested by @jsf9k, it's always best to minimize code duplication wherever possible. See #17 (comment) for more info.

Implementation notes

None

Acceptance criteria

• The same function is used to detect malformed targets in both nessus/nessus_importer.py and job_sink.py