Check to make sure cols & rows are INT

I've noticed that if the terminal is hidden and the screen experiences a resize trigger from a blur / focus, it can cause the dimensions to be undefined.  This creates an exception in Xterm.js that the values are not integers.  Figured it would be ok to just check before doing a resize.

addons/xterm-addon-fit/src/FitAddon.ts

@@ -33,7 +33,7 @@ export class FitAddon implements ITerminalAddon {
 
   public fit(): void {
     const dims = this.proposeDimensions();
-    if (!dims || !this._terminal) {
+    if (!dims || !this._terminal || isNaN(dims.cols) || isNaN(dims.rows)) {
       return;
     }