@thestinger checked the Linux x86_64 ABI, you've just checked for OSX.

This code is temporary until #13094 is fixed, in which case it will use memalign and these issues go away.

In the mean time, I do what I can with what I think I know, and that's that malloc is 16-byte aligned. If I'm wrong about this, a buildbot will fail and I'll need to rethink this.