Keep clang Address Sanitizer happy

Allocating 15 bytes with the 8-byte aligned memory at offset 8 of
15, would overflow the buffer, which would be fairly likely to
have no effect. But ASan notices this if you run it on AVX hardware,
unlike the Jenkins build which runs on SSE4.1. The good news is
that this fix is enough to make all the existing tests pass under
ASan on AVX.

Change-Id: I61ff11687709e096c70a162d3514227cb243561d