Sync a bit better the checks on the alignment value between
`MEM_lockfree_mallocN_aligned` and `MEM_guarded_mallocN_aligned`.
The only significant change, in `MEM_guarded_mallocN_aligned`, is the
usage of `ALIGNED_MALLOC_MINIMUM_ALIGNMENT` instead of 'magic value' `8`.
This should not have any effect on 64bits platforms, but on 32bits ones
the minimum alignment would be reduced from `8` to `4` now.
NOTE: we could also consider making these checks part of a utils
function, instead of duplicating them in the codebase.
461ee89b61