with the test case added by this patch.
The bug cannot be reproduced with the same test case for the main
5.3 tree because the backported fix for bug 59696 masks the 
problem that causes the crash in the mentioned test case. It's not
clear weather this fix masks this problem in all possible cases. 

Anyway the patch for bug 698882 introduced some inconsistent data
structures that could contain indirect references to deleted object.
It happened when two Item_equal objects were merged and the Item_field
list of the second object was joined to such list of the first object.
This operation required adjustment of the backward pointers in 
Item fields from the joined list. However the adjustment was missing 
and this caused crashes in the tree for mwl#128.

Now the backward pointers are set only when Item_equal items are
completely built and are not changed anymore.