vim-patch:8.0.1193: crash when wiping buffer after getbufinfo()

Problem:    Crash when wiping out a buffer after using getbufinfo().
            (Yegappan Lakshmanan)
Solution:   Remove b:changedtick from the buffer variables.

(Note: The test changes in vim-patch:8.0.1193 were already included.)

src/nvim/buffer.c

@@ -757,6 +757,8 @@ static void free_buffer(buf_T *buf)
   handle_unregister_buffer(buf);
   buf_free_count++;
   free_buffer_stuff(buf, true);
+  // b:changedtick uses an item in buf_T, remove it now.
+  tv_dict_item_remove(buf->b_vars, (dictitem_T *)&buf->changedtick_di);
   unref_var_dict(buf->b_vars);
   aubuflocal_remove(buf);
   tv_dict_unref(buf->additional_data);

src/nvim/buffer_defs.h

@@ -508,9 +508,9 @@ struct file_buffer {
   int b_changed;                // 'modified': Set to true if something in the
                                 // file has been changed and not written out.
 
-  /// Change identifier incremented for each change, including undo
+  /// Change-identifier incremented for each change, including undo.
   ///
-  /// This is a dictionary item used to store in b:changedtick.
+  /// This is a dictionary item used to store b:changedtick.
   ChangedtickDictItem changedtick_di;
 
   varnumber_T b_last_changedtick;       // b:changedtick when TextChanged or