Using move_field_offset() to move field pointer to other record before
calling pack()/unpack().


sql/log_event.cc:
  Using move_field_offset() to move the field pointer. It is necessary
  for unpack() and pack() of BIT fields to operate correctly.
  Adding debug printouts.
sql/sql_class.cc:
  Using move_field_offset() to move the field pointer. It is necessary
  for unpack() and pack() of BIT fields to operate correctly.
  Adding debug printouts to packing of update rows event.