Check video.error for null on MSE/EME demo

[hsivonen]

Steps to reproduce:

1. Navigate to https://developer.microsoft.com/en-us/microsoft-edge/testdrive/demos/eme/ in Firefox.
2. In the list of videos on the left, click any video that's not the currently-selected one.

Actual results: A message "TypeError: video.error is null" is logged.

Expected results: No error.

Additional info: The error location is line 729 in demo.js, which reads:

if (video.error.msExtendedCode) {

That line should null-check video.error before trying to check the msExtendedCode property on it.

[hsivonen]

CC @mohmoto

[dstorey]

CC @molant

[molant]

@mohmoto what do you want to do with this? As they say in https://github.com/webcompat/web-bugs/issues/1855 Firefox doesn't support WebM content in EME yet.

[mohmoto]

I'll fix this. It's pretty low pri though as it's in an error path.