PAGE SPLIT

page_rec_get_nth_const(): Map nth==0 to the page infimum.

btr_compress(adjust=TRUE): Add a debug assertion for nth>0. The cursor
should never be positioned on the page infimum.

btr_index_page_validate(): Add test instrumentation for checking the
return values of page_rec_get_nth_const() during CHECK TABLE, and for
checking that the page directory slot 0 always contains only one
record, the predefined page infimum record.

page_cur_delete_rec(), page_delete_rec_list_end(): Add debug
assertions guarding against accessing the page slot 0.

page_copy_rec_list_start(): Clarify a comment about ret_pos==0.

rb:1248 approved by Jimmy Yang