f_lavfi: reject filters with too many in/outs

The pad count serves as a minimum even if the filter has
dynamic inputs/outputs, so we should reject them here.

At least on FFmpeg 6.1 this results in no practical change
in the list accepted filters.

fixes: 8bf3fe7

filters/f_lavfi.c

@@ -987,6 +987,8 @@ static bool is_usable(const AVFilter *filter, int media_type)
     int nb_inputs  = avfilter_pad_count(filter->inputs),
         nb_outputs = avfilter_pad_count(filter->outputs);
 #endif
+    if (nb_inputs > 1 || nb_outputs > 1)
+        return false;
     bool input_ok = filter->flags & AVFILTER_FLAG_DYNAMIC_INPUTS;
     bool output_ok = filter->flags & AVFILTER_FLAG_DYNAMIC_OUTPUTS;
     if (nb_inputs == 1)