lavfi: replace empty input/output lists with null pointers

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavfilter/avfilter.c

@@ -302,6 +302,9 @@ static int pad_count(const AVFilterPad *pads)
 {
     int count;
 
+    if (!pads)
+        return 0;
+
     for(count = 0; pads->name; count ++) pads ++;
     return count;
 }