__crt_aligned_alloc_offset(): fix ov_index for backing allocation address

Wrong value of ov_index resulted in magic check failure, and refuse to
free() the memory allocated with __crt_aligned_alloc_offset().
Then the TLS segments of exited threads leaked.

Reported and tested by: glebius
Fixes: c29ee08204ce4106d4992474005c5f2fb7d5fbf1
Sponsored by: The FreeBSD Foundation
MFC after: 3 days