Third patch of the bug fix where the code for skipping events and for
executing events is factored out into three functions:
- shall_skip() to decide if the event shall be skipped and the
reason for it;
- do_apply_event(), where the event is applied to the database; and
- do_update_pos(), which updates the actual relay log position and
group positions.
Field_bit::set_default() did not check the bit_len, hence used the undefined
bit_ptr, causing a crash. The patch adds a check that bit_len > 0 before
following the bit_ptr.
