patch 9.1.1943: Memory leak with :breakadd expr

patch 9.1.1943: Memory leak with :breakadd expr

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: #18844

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>