contains a bad and a good copy

Clean up the InnoDB doublewrite buffer code.

buf_dblwr_init_or_load_pages(): Do not add empty pages to the buffer.

buf_dblwr_process(): Do consider changes to pages that are all zero.
Do not abort when finding a corrupted copy of a page in the doublewrite
buffer, because there could be multiple copies in the doublewrite buffer,
and only one of them needs to be good.