avfilter/scale_npp: check for buffer allocation failure

I totally did not forget to amend this to the previous patch...

libavfilter/vf_scale_npp.c

@@ -319,13 +319,7 @@ static int init_processing_chain(AVFilterContext *ctx, int in_width, int in_heig
         last_stage = i;
     }
 
-    if (last_stage < 0)
-    {
-        ctx->outputs[0]->hw_frames_ctx = av_buffer_ref(ctx->inputs[0]->hw_frames_ctx);
-        return 0;
-    }
-
-    ctx->outputs[0]->hw_frames_ctx = av_buffer_ref(s->stages[last_stage].frames_ctx);
+    ctx->outputs[0]->hw_frames_ctx = av_buffer_ref(s->stages[last_stage > 0 ? last_stage : 0].frames_ctx);
     if (!ctx->outputs[0]->hw_frames_ctx)
         return AVERROR(ENOMEM);