libc: Add missing object size check to qsort_s(3)

When sorting, both the C11 standard (ISO/IEC 9899:2011, K.3.6.3.2) and
the ISO/IEC JTC1 SC22 WG14 N1172 standard, does not define objects of
zero size as undefined behaviour. However Microsoft's cpp-docs does.

Add proper checks for this. Found while working on bsort(3).

Reviewed by: kib@ and emaste@
MFC after: 1 week
Sponsored by: NVIDIA Networking
Differential Revision: https://reviews.freebsd.org/D39687