POSIX mandates that swab do nothing when len < 0

PR:		kern/140690
Submitted by:	Jeremy Huddleston <jeremyhu@apple.com>
Approved by:	cperciva
MFC after:	2 weeks

lib/libc/string/swab.3

@@ -28,7 +28,7 @@
 .\"     @(#)swab.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd December 10, 2004
+.Dd March 4, 2012
 .Dt SWAB 3
 .Os
 .Sh NAME
@@ -54,6 +54,9 @@ swapping adjacent bytes.
 The argument
 .Fa len
 must be an even number.
+If
+.Fa len
+is less than zero, nothing will be done.
 .Sh SEE ALSO
 .Xr bzero 3 ,
 .Xr memset 3

lib/libc/string/swab.c

@@ -45,6 +45,8 @@ swab(const void * __restrict from, void * __restrict to, ssize_t len)
 	int n;
 	char *fp, *tp;
 
+	if (len <= 0)
+		return;
 	n = len >> 1;
 	fp = (char *)from;
 	tp = (char *)to;