Handle undefined cbox in Glyph metrics calculation

Fix issue with foliojs#306

src/glyph/Glyph.js

@@ -68,6 +68,7 @@ export default class Glyph {
     if (this._font.vmtx) {
       var {advance:advanceHeight, bearing:topBearing} = this._getTableMetrics(this._font.vmtx);
     } else {
+      if (typeof cbox === 'undefined' || cbox === null) { ({ cbox } = this); }
       var advanceHeight = Math.abs(this._font.ascent - this._font.descent);
       var topBearing = this._font.ascent - cbox.maxY;
     }