bug#45026: Heap corruption buffer overflow in bsd_probe

[Rich Felker]

Commit a5f69f396713ab8ac1e57458cbb9af552d2c1659 rearranged bsd.c's
bsd_probe function in a way that changed the meaning of the local
variable label, but left alone the call to alpha_bootblock_checksum,
thereby causing the checksum to take place over the wrong range of
bytes and be written 56 bytes past the end of the allocated memory.
The checksum call should probably just be removed as the results don't
seem to be used.

This was discovered via a bug report against the Apline Linux package,
https://gitlab.alpinelinux.org/alpine/aports/-/issues/12161. It
appears we just got really lucky catching this, as only one value well
beyond the end of the allocation is written. It turns out that 64+512
makes up exactly the size of musl/mallocng's next size class over 512,
576, and writing 8 bytes before that clobbers all the consistency
check at the end of the slot and the header of the next slot. However
valgrind also seems to catch the bug when running the test cases.