Bug 1792091 [Wayland] Assert when shm interface is missing r=emilio

There are some suspect crashes around Wayland SHM interface so let's make sure we have valid shm handle before we pass it to Wayland.
It covers the 'It can't happen' scenario which tends to happen despite of all premises.

Differential Revision: https://phabricator.services.mozilla.com/D157975

widget/gtk/WaylandBuffer.cpp

@@ -117,6 +117,7 @@ RefPtr<WaylandShmPool> WaylandShmPool::Create(
     return nullptr;
   }
 
+  MOZ_DIAGNOSTIC_ASSERT(aWaylandDisplay->GetShm(), "Missing Shm!");
   shmPool->mShmPool =
       wl_shm_create_pool(aWaylandDisplay->GetShm(), shmPool->mShmPoolFd, aSize);
   if (!shmPool->mShmPool) {