commit d09aec7a (MDEV-19940)
caused a regression. We made wait_lock_get_heap_no() return
uint16_t instead of ulint, and we mostly replaced the previous
magic value ULINT_UNDEFINED with 0. But, we failed to adjust
some assertions. Furthermore, 0 is a valid although rare value
for record locks. (Record locks can be temporarily stored on
page infimum in some operations that involve multiple leaf pages.)

Let us use 0xFFFF as the magic value. Valid heap numbers
are limited to less than 9362 = innodb_page_size/(5+1+1)
when using a minimal 1-byte PRIMARY KEY and a
secondary index on a NULL or '' column.