...in bitmap_intersect

m_cols_ai was accessed during the Delete event, however this field is only
related to Updates.

Moving it to Update_rows_event would require too much effort. So instead:
* Only access m_cols_ai in Update events (conditional branch is added in
  Rows_log_event::do_add_row_data)
* Clean up m_cols_ai operations in Rows_log_event constructor.
  m_cols_ai.bitmap is first set to NULL, indicating invalid event.
  Then it is initialized:
  -> For Update events, a new bitmap is created.
  -> For other events, debug mode, m_cols_ai.bitmap is set to 1, indicating
     that the value is correct, but it shouldn't be accessed. To make sure
     we'll have a failure, n_bits is also set to 1.
  -> In release mode, m_cols_ai mirrors m_cols, providing extra safety
     in production.