Add assertion SDL_SetVideoMode. NFC (emscripten-core#24177)

Also, avoid calling Browser.getCanvas() repeatedly withing this
function.

Author: sbc100

src/lib/libsdl.js

@@ -1484,15 +1484,19 @@ var LibrarySDL = {
   SDL_SetVideoMode__deps: ['$GL'],
   SDL_SetVideoMode__proxy: 'sync',
   SDL_SetVideoMode: (width, height, depth, flags) => {
+    var canvas = Browser.getCanvas();
+#if ASSERTIONS
+    assert(canvas, 'no canvas found');
+ #endif
+
     ['touchstart', 'touchend', 'touchmove',
      'mousedown', 'mouseup', 'mousemove',
      'mousewheel', 'wheel', 'mouseout',
      'DOMMouseScroll',
-    ].forEach((e) => Browser.getCanvas().addEventListener(e, SDL.receiveEvent, true));
+    ].forEach((e) => canvas.addEventListener(e, SDL.receiveEvent, true));
 
     // (0,0) means 'use fullscreen' in native; in Emscripten, use the current canvas size.
     if (width == 0 && height == 0) {
-      var canvas = Browser.getCanvas();
       width = canvas.width;
       height = canvas.height;
     }