Bug 1499758 - Fix potential null pointer deref in WebGLContext::GetVRFrame r=jgilbert

Differential Revision: https://phabricator.services.mozilla.com/D12480

--HG--
extra : moz-landing-system : lando

dom/canvas/WebGLContext.cpp

@@ -2328,6 +2328,9 @@ WebGLContext::GetVRFrame()
 already_AddRefed<layers::SharedSurfaceTextureClient>
 WebGLContext::GetVRFrame()
 {
+  if (!gl)
+      return nullptr;
+
   EnsureVRReady();
   /**
    * Swap buffers as though composition has occurred.
@@ -2337,9 +2340,6 @@ WebGLContext::GetVRFrame()
   BeginComposition();
   EndComposition();
 
-  if (!gl)
-      return nullptr;
-
   gl::GLScreenBuffer* screen = gl->Screen();
   if (!screen)
       return nullptr;