- When manually constructing a SEL_TREE for "t.key NOT IN(...)", take into account that 
  get_mm_parts may return a tree with type SEL_TREE::IMPOSSIBLE
- Added missing OOM checks
- Added comments


mysql-test/r/func_in.result:
  Testcase for BUG#19618
mysql-test/t/func_in.test:
  Testcase for BUG#19618