For queries like
update t1 set ... where <unique key> order by ... limit ...
we need to handle the fact that unique keys allow NULL
values, and hence can return more than one row.


sql/opt_range.cc:
  When the unique key has multiple key parts,
  check each key_part for nullability, rather than the first key part.
  (s/key->part ==/key_tree->part ==/)
  
  Also: revert the if() test, for better readability.