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

lib/libc/stdlib/qsort.3

@@ -32,7 +32,7 @@
 .\"     @(#)qsort.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd September 30, 2022
+.Dd April 19, 2023
 .Dt QSORT 3
 .Os
 .Sh NAME
@@ -260,7 +260,7 @@ The order of arguments to
 .Fa compar
 is different
 .It
-if
+If
 .Fa nmemb
 or
 .Fa size
@@ -270,7 +270,11 @@ or
 .Fa nmemb
 is not zero and
 .Fa compar
-is NULL, then the runtime-constraint handler is called, and
+is
+.Dv NULL
+or
+.Fa size
+is zero, then the runtime-constraint handler is called, and
 .Fn qsort_s
 returns an error.
 Note that the handler is called before

lib/libc/stdlib/qsort.c

@@ -247,6 +247,10 @@ qsort_s(void *a, rsize_t n, rsize_t es, cmp_t *cmp, void *thunk)
 			__throw_constraint_handler_s("qsort_s : cmp == NULL",
 			    EINVAL);
 			return (EINVAL);
+		} else if (es <= 0) {
+			__throw_constraint_handler_s("qsort_s : es <= 0",
+			    EINVAL);
+			return (EINVAL);
 		}
 	}