Merge pull request #232 from google/reland-snprintf

Reland snprintf fix

Author: battre

cpp/src/validating_util.cc

@@ -84,8 +84,8 @@ bool UnwrapHeader(const char* header_prefix,
 void ValidatingUtil::Wrap(time_t timestamp, std::string* data) {
   assert(data != nullptr);
   char timestamp_string[2 + 3 * sizeof timestamp];
-  int size =
-      std::sprintf(timestamp_string, "%ld", static_cast<long>(timestamp));
+  int size = std::snprintf(timestamp_string, sizeof(timestamp_string), "%ld",
+                                             static_cast<long>(timestamp));
   assert(size > 0);
   assert(size < sizeof timestamp_string);
   (void)size;