Fix an out-of-bounds write when a zero-length buffer is passed.

Found with ttyname_test and CHERI bounds checking.

Reviewed by:	emaste
Obtained from:	CheriBSD
MFC after:	1 week
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D10377

lib/libc/gen/ttyname.c

@@ -61,6 +61,10 @@ ttyname_r(int fd, char *buf, size_t len)
 {
 	size_t used;
 
+	/* Don't write off the end of a zero-length buffer. */
+	if (len < 1)
+		return (ERANGE);
+
 	*buf = '\0';
 
 	/* Must be a terminal. */