This bug was introduced in
commit 7ae21b18
(the main commit of MDEV-12353).

page_cur_insert_rec_low(): Before entering the comparison loop, make sure
that the range does not exceed c_end already at the start of the loop.
The loop is only comparing for pointer equality, and that condition
cdm == c_end would never hold if the end was already exceeded in
the beginning. Also, skip the comparison altogether if we could find
at most 2 equal bytes.

PageBulk::insertPage(): Apply a similar change. It seems that this
code was correct, because the loop checks for cdm < c_end.