[Fuchsia] Fix crash in SysmemBufferCollection destructor.

Previously SysmemBufferCollection could crash if it failed to initialize
(e.g. due to Allocator interface missing).

Change-Id: I72f83ebe7ad858ab908ad42fb08aeff895327c67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614001
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660928}

ui/ozone/platform/scenic/sysmem_buffer_collection.cc

@@ -362,7 +362,8 @@ SysmemBufferCollection::~SysmemBufferCollection() {
                                      nullptr);
   }
 
-  collection_->Close();
+  if (collection_)
+    collection_->Close();
 
   if (on_deleted_)
     std::move(on_deleted_).Run();