Crash on invalid UTF-8 input

[chrysn]

$ pretty2diag.rb
.../3.0.0/gems/cbor-diag-0.7.6/lib/cbor-diagnostic.rb:73:in `to_json': source sequence is illegal/malformed utf-8 (JSON::GeneratorError)
        from .../3.0.0/gems/cbor-diag-0.7.6/lib/cbor-diagnostic.rb:73:in `cbor_diagnostic'
        from .../3.0.0/gems/cbor-diag-0.7.6/lib/cbor-diagnostic.rb:83:in `block in cbor_diagnostic'
        from .../3.0.0/gems/cbor-diag-0.7.6/lib/cbor-diagnostic.rb:83:in `map'
        from .../3.0.0/gems/cbor-diag-0.7.6/lib/cbor-diagnostic.rb:83:in `cbor_diagnostic'
        from .../3.0.0/gems/cbor-diag-0.7.6/lib/cbor-diagnostic-helper.rb:32:in `cbor_diagnostic_output'
        from .../3.0.0/gems/cbor-diag-0.7.6/bin/pretty2diag.rb:17:in `<top (required)>'
        from .../3.0.0/bin/pretty2diag.rb:25:in `load'
        from .../3.0.0/bin/pretty2diag.rb:25:in `<main>'

Originally reported by @Cody119 at cbor/cbor.github.io#83