Ensure minsize >= sizeof(SH_LIST)

The sh_add_to_list function will overwrite subsequent slots in the free list
for small allocations.  This causes a segmentation fault if the writes goes
off the end of the secure memory.  I've not investigated if this problem
can overwrite memory without the segmentation fault, but it seems likely.

This fix limits the minsize to the sizeof of the SH_LIST structure (which
also has a side effect of properly aligning the pointers).

The alternative would be to return an error if minsize is too small.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2657)

(cherry picked from commit 70e14ffb)