Add a check for undefined layer

[amishas157]

Hi, I ran into following issue while using vt2geojson.

/Users/AmishaSingla/Documents/osm-compare/node_modules/@mapbox/vt2geojson/index.js:78
        for (var i = 0; i < layer.length; i++) {
                                 ^

TypeError: Cannot read property 'length' of undefined
    at /Users/AmishaSingla/Documents/osm-compare/node_modules/@mapbox/vt2geojson/index.js:78:34
    at Array.forEach (native)
    at readTile (/Users/AmishaSingla/Documents/osm-compare/node_modules/@mapbox/vt2geojson/index.js:76:12)
    at Request._callback (/Users/AmishaSingla/Documents/osm-compare/node_modules/@mapbox/vt2geojson/index.js:41:13)
    at Request.self.callback (/Users/AmishaSingla/Documents/osm-compare/node_modules/request/request.js:188:22)
    at emitTwo (events.js:87:13)
    at Request.emit (events.js:172:7)
    at Request.<anonymous> (/Users/AmishaSingla/Documents/osm-compare/node_modules/request/request.js:1171:10)
    at emitOne (events.js:77:13)
    at Request.emit (events.js:169:7)

Here is code snippet:

var retrieveTileData = function (x, y, z, callback) {
  var url = 'http://a.tiles.mapbox.com/v4/mapbox.mapbox-streets-v7/' + z + '/' + x + '/' + y + '.mvt?access_token=pk.eyJ1IjoiYW1pc2hhIiwiYSI6ImNpcWt1bWc4bjAzOXNmdG04bng4dHVhZ3EifQ.bJK6rpjNmiP3kW2pROdScg';
  vt2geojson({
    uri: url,
    layer: ['building', 'road', 'waterway']
  }, function (err, result) {
    if (err) return callback(err, null);
    return callback(null, result);
  });
};

Looked like the layer didn't exist for the waterway type features. This PR might help in resolving the issue.

cc @mapsam

[mapsam]

@amishas157 great catch! Any interest in adding a test for this?

[amishas157]

@mapsam , added the test here: e857da7

[mapsam]

Marvelous, thanks so much @amishas157!