Fix panic when parsing an IotaDID with more than 2 method id segments

[PhilippGackstatter]

Description of change

This currently panics: IotaDID::parse("did:iota:custom:random:H3C2AVvLMv6gmMNam3uVAjZpfkcJCwDwnZn6z3wXmqPV"), due to entering unreachable code in Segments::network. To fix this, IotaDID::check_method_id should return an error when the number of segments of the method id is > 2 instead of > 3.

Type of change

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• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Added a test.

Change checklist

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• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[cycraig]

Looks good to me.