Bug 1483772 - Never have mAsyncTask be non-null when we cannot paint. r=nical

In bug 1482415 a special check was added for the case where we fail to allocate
a buffer and started an async task. This papered over one crash for another
as ClientPaintedLayer also assumes that if there is an async task there is
a capture. It'd be best to just null out mAsyncTask and keep those checks
as is.

Differential Revision: https://phabricator.services.mozilla.com/D3520

gfx/layers/client/ContentClient.cpp

@@ -237,12 +237,14 @@ ContentClient::BeginPaint(PaintedLayer* aLayer,
                         << dest.mBufferRect.Width() << ", "
                         << dest.mBufferRect.Height();
       }
+      result.mAsyncTask = nullptr;
       Clear();
       return result;
     }
 
     if (!newBuffer->Lock(writeMode)) {
       gfxCriticalNote << "Failed to lock new back buffer.";
+      result.mAsyncTask = nullptr;
       Clear();
       return result;
     }
@@ -295,7 +297,7 @@ ContentClient::BeginPaint(PaintedLayer* aLayer,
 void
 ContentClient::EndPaint(PaintState& aPaintState, nsTArray<ReadbackProcessor::Update>* aReadbackUpdates)
 {
-  if (aPaintState.mAsyncTask && mBuffer) {
+  if (aPaintState.mAsyncTask) {
     aPaintState.mAsyncTask->mCapture = mBuffer->EndCapture();
   }
 }