Bug reproduction for fast-json-stringify

index.js

// Node version: v8.0.0
// fast-json-stringify-version version: 0.15.1
// Windows 10, 64 bit

const fastJson = require('fast-json-stringify');

const goodSchema = {
  title: 'Example Schema',
  type: 'object',
  properties: {
    nickname: {
      $ref: 'strings#'
    }
  },
  patternProperties: {
    'num': {
      $ref: 'numbers#/definitions/num'
    }
  }
}

const badSchema = {
  title: 'Example Schema',
  type: 'object',
  properties: {
    nickname: {

      // note that the hash is omitted from this; this is valid JSON schema
      // see: https://spacetelescope.github.io/understanding-json-schema/structuring.html#the-id-property
      $ref: 'strings' 

    }
  },
  patternProperties: {
    'num': {
      $ref: 'numbers#/definitions/num'
    }
  }
}

const externalSchema = {
  numbers: {
    definitions: {
      num: {
        type: 'object',
        properties: {
          int: {
            type: 'integer'
          }
        }
      }
    }
  },
  strings: {
    type: 'string'
  }
}

// this works as expected
const stringify = fastJson(goodSchema, { schema: externalSchema })

// this crashes
const stringify = fastJson(schema, { schema: externalSchema })

/*

## crash log

<path>\fast-json-stringify-issue>node ./index.js
<path>\fast-json-stringify-issue\node_modules\fast-json-stringify\index.js:354
  var walk = ref[1].split('/')
                   ^

TypeError: Cannot read property 'split' of undefined
    at refFinder (<path>\fast-json-stringify-issue\node_modules\fast-json-stringify\index.js:354:20)
    at Object.keys.forEach (<path>\fast-json-stringify-issue\node_modules\fast-json-stringify\index.js:387:32)
    at Array.forEach (native)
    at buildObject (<path>\fast-json-stringify-issue\node_modules\fast-json-stringify\index.js:377:40)
    at build (<path>\fast-json-stringify-issue\node_modules\fast-json-stringify\index.js:58:14)
    at Object.<anonymous> (<path>\fast-json-stringify-issue\index.js:36:19)
    at Module._compile (module.js:569:30)
    at Object.Module._extensions..js (module.js:580:10)
    at Module.load (module.js:503:32)
    at tryModuleLoad (module.js:466:12)

*/