shred: don't infloop upon negative size

* src/shred.c (main): With the preceding change, shred -s-2 FILE
would write 64KB blocks forever -- or until disk full. This change
makes shred reject a negative size.
* tests/misc/shred-negative.sh: New file.
* tests/local.mk (all_tests): Add it.

src/shred.c

@@ -1256,8 +1256,8 @@ main (int argc, char **argv)
 
         case 's':
           {
-            intmax_t tmp;
-            if ((xstrtoimax (optarg, NULL, 0, &tmp, "cbBkKMGTPEZY0")
+            uintmax_t tmp;
+            if ((xstrtoumax (optarg, NULL, 0, &tmp, "cbBkKMGTPEZY0")
                  != LONGINT_OK)
                 || OFF_T_MAX < tmp)
               {

tests/local.mk

@@ -313,6 +313,7 @@ all_tests =					\
   tests/misc/sha384sum.pl			\
   tests/misc/sha512sum.pl			\
   tests/misc/shred-exact.sh			\
+  tests/misc/shred-negative.sh			\
   tests/misc/shred-passes.sh			\
   tests/misc/shred-remove.sh			\
   tests/misc/shuf.sh				\

tests/misc/shred-negative.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+# Exercise shred -s-3 FILE
+
+# Copyright (C) 2014 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ shred
+
+echo 'shred: -2: invalid file size' > exp || framework_failure_
+echo 1234 > f || framework_failure_
+
+shred -s-2 f 2>err && fail=1
+compare exp err || fail=1
+
+Exit $fail