there will always be enough space for two node pointer records in an
empty B-tree page.  This was reported as Mantis issue #73.

page_zip_rec_needs_ext(): Add the parameter n_fields, for accurate
estimation of the compressed size of the data dictionary information.
Given that this function is only invoked for records on leaf pages,
require that there be enough space for one record in the compressed
page.  We check elsewhere that there will be enough room for two node
pointer records on higher-level pages.

btr_cur_optimistic_insert(): Ensure that there will be enough room for
two node pointer records on an empty non-leaf page.  The rule for
leaf-page records will be enforced by the callers of
page_zip_rec_needs_ext().

btr_cur_pessimistic_insert(): Remove the insufficient check that the
leaf page record should be compressible by itself.  Instead, now we
require that two node pointer records fit on a non-leaf page, and one
record will fit in uncompressed form on the leaf page.

page_zip_write_header(), page_zip_write_rec(): Re-enable the debug
assertions that were violated by the insufficient check in
btr_cur_pessimistic_insert().

innodb_bug36172.test: Use a larger compressed page size.