Fix overly-conservative overflow checks on mulitplications and add checks

on additions.  This allows scan_scaled to work up to +/-LLONG_MAX (LLONG_MIN
will still be flagged as a range error).  ok millert@

src/lib/libutil/fmt_scaled.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: fmt_scaled.c,v 1.15 2017/03/15 05:25:56 dtucker Exp $	*/
+/*	$OpenBSD: fmt_scaled.c,v 1.16 2017/03/16 02:40:46 dtucker Exp $	*/
 
 /*
  * Copyright (c) 2001, 2002, 2003 Ian F. Darwin.  All rights reserved.
@@ -121,22 +121,30 @@ scan_scaled(char *scaled, long long *result)
 				/* ignore extra fractional digits */
 				continue;
 			fract_digits++;		/* for later scaling */
-			if (fpart >= LLONG_MAX / 10) {
+			if (fpart > LLONG_MAX / 10) {
 				errno = ERANGE;
 				return -1;
 			}
 			fpart *= 10;
+			if (i > LLONG_MAX - fpart) {
+				errno = ERANGE;
+				return -1;
+			}
 			fpart += i;
 		} else {				/* normal digit */
 			if (++ndigits >= MAX_DIGITS) {
 				errno = ERANGE;
 				return -1;
 			}
-			if (whole >= LLONG_MAX / 10) {
+			if (whole > LLONG_MAX / 10) {
 				errno = ERANGE;
 				return -1;
 			}
 			whole *= 10;
+			if (i > LLONG_MAX - whole) {
+				errno = ERANGE;
+				return -1;
+			}
 			whole += i;
 		}
 	}