scale_npp: fix passthrough mode

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/vf_scale_npp.c

@@ -337,7 +337,7 @@ static int init_processing_chain(AVFilterContext *ctx, int in_width, int in_heig
     }
 
     if (last_stage < 0)
-        return AVERROR_BUG;
+        return 0;
     ctx->outputs[0]->hw_frames_ctx = av_buffer_ref(s->stages[last_stage].frames_ctx);
     if (!ctx->outputs[0]->hw_frames_ctx)
         return AVERROR(ENOMEM);