range: T6744: fix a crash when validating malformed ranges

[dmbaturin]

Change Summary

Fix a crash that occurs when trying to validate good-looking but wrong addresses:

vyos@vyos# ipaddrcheck --is-ipv4-range 192.0.2.400-192.0.2.800 && echo y
Segmentation fault

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Other (please describe):

Proposed changes

The cause was that individual components weren't validated properly before trying to convert them to inaddr structures and manipulate them.

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly