Fix #213, updated CFDP class validation

[havencarlson]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to [email protected] and copied [email protected].

Describe the contribution
Fix #213, add validation to CF_CmdTxFile() and CF_CmdPlaybackDir() to prevent negative CFDP class values.

Testing performed
Unit testing

Expected behavior changes
Reject command if CFDP class is negative

System(s) tested on

• OS: Ubuntu 18.04

Contributor Info - All information REQUIRED for consideration of pull request
Haven Carlson - NASA

[dzbaker]

CCB 3 Aug 2022: Check that it is equal to 1 or 2 in validation.

[jphickey]

Here's another suggestion - define CF_CLASS_MAX in the enum, and check that it is not greater than that. That's the way I often see enums validated, as it avoids the assumption that a certain label is the "last one"