add a termination condition

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@16422 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavutil/tree.c

@@ -128,9 +128,11 @@ void *av_tree_insert(AVTreeNode **tp, void *key, int (*cmp)(void *key, const voi
 }
 
 void av_tree_destroy(AVTreeNode *t){
+    if(t){
     av_tree_destroy(t->child[0]);
     av_tree_destroy(t->child[1]);
     av_free(t);
+    }
 }
 
 #if 0