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

vm/os_macos.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 "vm/os.h"
 
 #include <errno.h>
 #include <limits.h>
 #include <mach/mach.h>
 #include <mach/clock.h>
@@ skipping 110 matching lines @@
 int OS::SNPrint(char* str, size_t size, const char* format, ...) {
   va_list args;
   va_start(args, format);
   int retval = VSNPrint(str, size, format, args);
   va_end(args);
   return retval;
 }
 
 
 int OS::VSNPrint(char* str, size_t size, const char* format, va_list args) {
-  return vsnprintf(str, size, format, args);
+  int retval = vsnprintf(str, size, format, args);
+  if (retval < 0) {
+    FATAL1("Fatal error in OS::VSNPrint with format '%s'", format);
+  }
+  return retval;
 }
 
 
 void OS::PrintErr(const char* format, ...) {
   va_list args;
   va_start(args, format);
   VFPrint(stderr, format, args);
   va_end(args);
 }
 
@@ skipping 15 matching lines @@
 void OS::Abort() {
   abort();
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart