This fixes a regression that only affects debug builds, caused by
commit 48192f96 which is necessary
preparation for MDEV-11369 instant ADD COLUMN. (Although that is a
10.3 task, to ease merges between 10.2 and 10.3, this change that
improves debug checks was pushed to 10.2 already.)

Unlike btr_pcur_restore_position(), rtr_cur_restore_position()
can create a search tuple out of a non-leaf page record. So,
we must pass 'bool leaf' parameter to dict_index_build_data_tuple().

dict_index_build_data_tuple(): Add a debug-only parameter 'bool leaf'.

rec_copy_prefix_to_dtuple(): Make the parameter debug-only.

row_sel_get_clust_rec_for_mysql(): In the debug code for spatial index,
remove an unnecessary call to buf_page_get_gen(), and use the already
latched block directly.