avfilter: allow freeing NULL.

this way avfilter_free() can be called without NULL checks.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/avfilter.c

@@ -834,6 +834,9 @@ void avfilter_free(AVFilterContext *filter)
     int i;
     AVFilterLink *link;
 
+    if (!filter)
+        return;
+
     if (filter->filter->uninit)
         filter->filter->uninit(filter);