Fix sporadic CUDA launch failures with Cycles viewport denoising

Sometimes the viewport buffer size is zero for a frame, which caused the denoising task to also try to
launch CUDA kernels with a launch size of zero, which in turn failed with a CUDA error. This patch
prevents launches from occuring in this case, similar to how it is handled in `copy_to_display_buffer`.

intern/cycles/render/session.cpp

@@ -1128,6 +1128,10 @@ void Session::denoise()
   else {
     assert(buffers);
 
+    if (tile_manager.state.buffer.width == 0 || tile_manager.state.buffer.height == 0) {
+      return; /* Avoid empty launches. */
+    }
+
     /* Wait for rendering to finish. */
     device->task_wait();