RearPort position count should never be less than the number of mapped FrontPorts

[jeremystretch]

Environment

• Python version: 3.6.9
• NetBox version: 2.9.4

Steps to Reproduce

1. Create a rear port with 4 positions
2. Create four front ports mapped to it
3. Modify the rear port position count to 3 and save

Expected Behavior

This should trigger a validation error, as there are four front ports mapped to it.

Observed Behavior

The rear port is saved without error, leaving the database and cable path in an inconsistent state. (Attempting to modify the orphaned front port yields a validation error as expected.)