Fix #50, Mark end of valid MsgId subscriptions

[skliper]

Describe the contribution
Fix #50

• Added a TO_UNUSED entry at the end of the subscription list
• Added break in subscription loop when TO_UNUSED found

Testing performed
Normal run, confirmed no 0x0 duplicate subscription message, confirmed nominal hk telemetry still works with GroundSystem

Expected behavior changes
No more subscriptions on the unused table slots (no MsgId 0 subscriptions).

System(s) tested on

• Hardware: cFS Dev 3
• OS: Ubuntu 18.04
• Versions: bundle

Additional context
None

Third party code
None

Contributor Info - All information REQUIRED for consideration of pull request
Jacob Hageman - NASA/GSFC

[astrogeco]

CCB 2020-07-29, Suggestion: Consolidate unused entries in table. Some potential issues with equality check using TO_UNUSED

[skliper]

Switch to use the compare msgid function.

[skliper]

Updated to use MsgId Equal.

[jphickey]

I'm OK with it now using CFE_SB_MsgId_Equal()....

HOWEVER - one difference is that the previous implementation did allow "holes" in the list i.e. a valid entry, a TO_UNUSED entry, another valid entry, etc. This could have been used for "reserved" entries that could be turned on/off based on whether an app was loaded, without changing the positions of other entries.

This will now stop at the first TO_UNUSED entry - so holes/reserved slots are no longer possible.

As long as folks are OK with that, I'm fine with it -- I don't see why someone would need to leave gaps/holes, just that it is a change worth noting.

[skliper]

Good note, and no holes approach is fine by me.