The problem: base_list::remove didn't modify base_list::last when removing 
the last list element.
The fix: If we remove the last element, find the element before it (by walking
from the beginning of the list) and set base_list::last accordingly.

The list gets corrupted in both 4.0 and 4.1. There are no visible problems in 
current 4.1 because current 4.1 doesn't call where_cond->fix_fields() after 
constant propagation step.