Bug#34946811: Failure in Item_ref::walk/Item_cond::fix_fields

Isolate some simple refactoring before the actual bugfix

- Simplify return values of transformer functions on subquery objects.
  Since the value RES_REDUCE is never used, a regular false/true return
  is sufficient.

- Implement property function has_tables() on class Query_block and
  use it wherever we distinguish a query block without tables.

- More extensive use of Query_block property functions is_grouped(),
  has_windows(), is_explicitly_grouped(), has_limit().

- Explicit check on nullptr for pointers to items.

- Error checks wherever needed.

Change-Id: Id3ec65718ce39effa8bc15d92a5dbb160b0d68d7

Author: roylyseng