GlyphSet.prototype.get can return undefined (#755)

* GlyphSet.prototype.get can return undefined

Calling `font.glyphs.get(9999)` should return `undefined` if the glyph doesn't exist.

* Simplify low memory mode check

Author: forresto

src/glyphset.mjs

@@ -62,11 +62,11 @@ if(typeof Symbol !== 'undefined' && Symbol.iterator) {
 
 /**
  * @param  {number} index
- * @return {opentype.Glyph}
+ * @return {opentype.Glyph | undefined}
  */
 GlyphSet.prototype.get = function(index) {
     // this.glyphs[index] is 'undefined' when low memory mode is on. glyph is pushed on request only.
-    if (this.glyphs[index] === undefined) {
+    if (this.font._push && this.glyphs[index] === undefined) {
         this.font._push(index);
         if (typeof this.glyphs[index] === 'function') {
             this.glyphs[index] = this.glyphs[index]();