Don't allow OS::SNPrint or OS::VSNPrint to return negative values.

vm/os_test.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/globals.h"
 #include "vm/os.h"
 #include "vm/unit_test.h"
 
@@ skipping 20 matching lines @@
   EXPECT_EQ(3, length);
   EXPECT_STREQ("fo", buffer);
   length = OS::SNPrint(buffer, 256, "%s%c%d", "foo", 'Z', 42);
   EXPECT_EQ(6, length);
   EXPECT_STREQ("fooZ42", buffer);
   length = OS::SNPrint(NULL, 0, "foo");
   EXPECT_EQ(3, length);
 }
 
 
+// This test is expected to crash when it runs.
+UNIT_TEST_CASE(SNPrint_BadArgs) {
+  int width = kMaxInt32;
+  int num = 7;
+  OS::SNPrint(NULL, 0, "%*d%*d", width, num, width, num);
+}
+
+
 UNIT_TEST_CASE(OsFuncs) {
   EXPECT(Utils::IsPowerOfTwo(OS::ActivationFrameAlignment()));
   EXPECT(Utils::IsPowerOfTwo(OS::PreferredCodeAlignment()));
   int procs = OS::NumberOfAvailableProcessors();
   EXPECT_LE(1, procs);
   EXPECT_LT(0U, OS::GetStackSizeLimit());
 }
 
 }  // namespace dart