Be resilient against unexpected end of JSON input

[mcollina]

SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at Parser._flush (/Users/matteo/.nvm/versions/node/v10.14.2/lib/node_modules/clinic/node_modules/@nearform/trace-events-parser/index.js:58:24)
    at Parser.prefinish (_stream_transform.js:141:10)
    at Parser.emit (events.js:182:13)
    at prefinish (_stream_writable.js:630:14)
    at finishMaybe (_stream_writable.js:638:5)
    at endWritable (_stream_writable.js:649:3)
    at Parser.Writable.end (_stream_writable.js:589:5)
    at ReadStream.onend (_stream_readable.js:628:10)
    at Object.onceWrapper (events.js:273:13)