lavfi: do not segfault on NULL passed to avfilter_get_by_name()

libavfilter/avfilter.c

@@ -273,6 +273,9 @@ AVFilter *avfilter_get_by_name(const char *name)
 {
     AVFilter *f = NULL;
 
+    if (!name)
+        return NULL;
+
     while ((f = avfilter_next(f)))
         if (!strcmp(f->name, name))
             return f;