I have removed the incorrect code in master and 3.6, this unblocks testing debug build on OpenBSD. I don't think it is worth to backport this change to 3.5 and 3.4, since the bug affects only debug build.

Here is a patch which perhaps is an alternative to PR 4119, but without disturbing the heap allocator. It erases just few bytes at the begin and at the end of the allocated block (including the header and the trailer), and saves erased bytes in small local buffer. It does this even if the block is not reallocated in-place. Hence calling free() on reallocated block will be detected. Reading from the reallocated block will get erased bytes at the start and the end, this is likely will cause to loud failure too. I have tested in on OpenBSD.

But I'm not sure that it is worth to apply this patch. Left it on to you Victor.