patch 8.2.3665: cannot use a lambda for 'tagfunc'

Problem:    Cannot use a lambda for 'tagfunc'.
Solution:   Use 'tagfunc' like 'opfunc'. (Yegappan Lakshmanan, closes #9204)
diff --git a/src/buffer.c b/src/buffer.c
index 2983ca9..e2732de 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2344,6 +2344,7 @@
     clear_string_option(&buf->b_p_tc);
 #ifdef FEAT_EVAL
     clear_string_option(&buf->b_p_tfu);
+    free_callback(&buf->b_tfu_cb);
 #endif
     clear_string_option(&buf->b_p_dict);
     clear_string_option(&buf->b_p_tsr);