scale_npp: explicitly set the output frames context for passthrough mode

This is no longer done automatically for filters marked as
hwframe-aware.

libavfilter/vf_scale_npp.c

@@ -336,9 +336,11 @@ static int init_processing_chain(AVFilterContext *ctx, int in_width, int in_heig
         last_stage = i;
     }
 
-    if (last_stage < 0)
-        return 0;
-    ctx->outputs[0]->hw_frames_ctx = av_buffer_ref(s->stages[last_stage].frames_ctx);
+    if (last_stage >= 0)
+        ctx->outputs[0]->hw_frames_ctx = av_buffer_ref(s->stages[last_stage].frames_ctx);
+    else
+        ctx->outputs[0]->hw_frames_ctx = av_buffer_ref(ctx->inputs[0]->hw_frames_ctx);
+
     if (!ctx->outputs[0]->hw_frames_ctx)
         return AVERROR(ENOMEM);