Hard crash when setting font with invalid size

[zbjornson]

Steps to Reproduce

Since 96bdd4f, this will cause a hard crash from an empty MaybeLocal because parseFont returns undefined.

const {createCanvas} = require(".");
const ctx = createCanvas(10,10).getContext("2d");
ctx.font = "bold undefinedpx Arial";

Crashing at

node-canvas/src/CanvasRenderingContext2d.cc

Line 2455 in 7de2441

Local<Object> font = Nan::To<Object>(parsed).ToLocalChecked();

Your Environment

• Version of node-canvas (output of npm list canvas or yarn list canvas): 96bdd4f
• Environment (e.g. node 4.2.0 on Mac OS X 10.8): Win10, Node 10