fix(marker-display): error when changing resource

Fix #12 which occurred when loading resource after starting playback. This bug
didn't cause any component malfunctions, apart from displaying errors in the
console.

- adds a guard clause to `updateMarker` to update the marker only if
`getClientRects` is not empty

Author: amtins

src/markers/src/marker-display.js

@@ -68,6 +68,8 @@ class MarkerDisplay extends videojs.getComponent('component') {
    * @param {string} cssVar - The CSS variable name.
    */
   updateMarker(time = 0, cssVar) {
+    if(!this.parentComponent_.el().getClientRects().length) return;
+
     const duration = this.player().duration();
     const markersElWidth = this.parentComponent_.el().getClientRects()[0].width;
     const markerOffsetLeft = this.el().offsetLeft;