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

vm/os_win.cc

Index: vm/os_win.cc
===================================================================
--- vm/os_win.cc	(revision 9556)
+++ vm/os_win.cc	(working copy)
@@ -155,7 +155,11 @@
 
 int OS::VSNPrint(char* str, size_t size, const char* format, va_list args) {
   if (str == NULL || size == 0) {
-    return _vscprintf(format, args);
+    int retval = _vscprintf(format, args);
+    if (retval < 0) {
+      FATAL1("Fatal error in OS::VSNPrint with format '%s'", format);
+    }
+    return retval;
   }
   va_list args_copy;
   va_copy(args_copy, args);
@@ -168,6 +172,9 @@
     va_list args_retry;
     va_copy(args_retry, args);
     written = _vscprintf(format, args_retry);
+    if (written < 0) {
+      FATAL1("Fatal error in OS::VSNPrint with format '%s'", format);
+    }
     va_end(args_retry);
   }
   // Make sure to zero-terminate the string if the output was