Don't use AddSubstring for external natives

src/utils.h

Index: src/utils.h
diff --git a/src/utils.h b/src/utils.h
index d5685c98207d3a2211919f4a0dbbafef2b980304..525c6f87d3d96fbd8504a00474981675c1c1f74d 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -863,7 +863,7 @@ class SimpleStringBuilder {
   // compute the length of the input string.
   void AddString(const char* s);
 
-  // Add the first 'n' characters of the given string 's' to the
+  // Add the first 'n' characters of the given 0-terminated string 's' to the
   // builder. The input string must have enough characters.
   void AddSubstring(const char* s, int n);