Add assert statements to test (in debug mode) if the VPChannelID is accesses while uninitialised.

This extends the change made in !542 (merged) so that all access to VPChannelID where the default value is still set (now max unsigned int) will case an assert failure and crash in debug mode. The code should be removed from the optimised path so unaffected.

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Validated by

• Core Software
• RTA
• Simulation