When the optimizer creates items out of other items it does
not have to call the fix_fields method. Usually in these
cases it calls quick_fix_field() that just marks the 
created item as fixed. If the created item is an Item_func
object then calling quick_fix_field() works fine if the
arguments of the created functional item are already fixed.
Otherwise some unfixed nodes remain in the item tree and
it triggers an assertion failure whenever the item is 
evaluated.

Fixed the problem by making the method quick_fix_field
virtual and providing an implementation for the class
Item_func objects that recursively calls the method
for unfixed arguments of any functional item.