MDEV-23672 (commit 7eda5561)
introduced a regression that can corrupt not only undo log pages,
but anything that resides in the InnoDB buffer pool.

trx_undo_left(): Add debug assertions for the assumptions.
If the pointer is out of bounds, we will return a positive
number, not a negative one. Thus, once a page overflow occurs,
further overflow to adjacent pages will be allowed.
This allows us to remove some more relaxed debug assertions
from some callers.

trx_undo_log_v_idx(): Correctly calculate the size limit.