Problem: Memory leak with :breakadd expr
Solution: Free debug_oldval and debug_newval before assigning to them.
Verify the existing (though confusing) :breakadd expr behavior
(zeertzjq).
It seems that :breakadd expr doesn't work as documented at all. This PR
only fixes the memory leak. The tests are for the existing behavior.
closes: vim/vim#18844a474de64df