unintentional undefined behaviore in test-memwipe.c
While working on #22839 (moved), snoek found a bug in test-memwipe.c:
The last test, test_memwipe, segfaulted. This only happened with Rust built in. It turns out this was caused by the uninitialized buf in check_heap_buffer being smaller than the mem addresses being scanned. I know we're doing some dirty stuff there, but I don't think trying to read past the length of the buffer was intended. At least to me it seems fair enough for the program to segfault. I put in the obvious fix, which might be horribly wrong.
It looks fine to me, so I'm going to give it a bug number and backport it.