BUG/MEDIUM: shctx: make sure to keep all blocks aligned

The blocksize and the extra field are not necessarily aligned on a
machine word. This can result in crashing an align-sensitive machine
when initializing the shctx area. Let's round both sizes up to a pointer
size to make this safe everywhere.

This fixes issue #512. This should be backported as far as 1.8.
1 file changed