add a termination condition

Originally committed as revision 16422 to svn://svn.ffmpeg.org/ffmpeg/trunk

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