Fix uninitialized ReadbackWorker pointer.

This was causing flaky crashes/hangs when doing screen recording.

Bug: 266833942
Test: screen recording
Change-Id: I4ce1110fd8868e8bd944654360cb423e148c91a4
diff --git a/stream-servers/FrameBuffer.h b/stream-servers/FrameBuffer.h
index d753ebe..89d7db4 100644
--- a/stream-servers/FrameBuffer.h
+++ b/stream-servers/FrameBuffer.h
@@ -792,7 +792,7 @@
         }
     };
     std::map<uint32_t, onPost> m_onPost;
-    gfxstream::ReadbackWorker* m_readbackWorker;
+    gfxstream::ReadbackWorker* m_readbackWorker = nullptr;
     android::base::WorkerThread<Readback> m_readbackThread;
     std::atomic_bool m_readbackThreadStarted = false;