directrenderer: handle pull callback failure

If the frameBufferRequested signal fails to set the AVFrame data
pointer, return null.

Change-Id: Iefb0d4670ca5e94c6ace2759fc941b9af39e682e

src/directrenderer.cpp

@@ -68,6 +68,10 @@ public:
         // Important: Subscription to this signal MUST be synchronous(Qt::DirectConnection).
         Q_EMIT parent_->frameBufferRequested(frameBufferPtr->avframe.get());
 
+        if (frameBufferPtr->avframe->data[0] == nullptr) {
+            return nullptr;
+        }
+
         return std::move(frameBufferPtr);
     };