From 9c846bd52eed100d06de5f23c121011328575c11 Mon Sep 17 00:00:00 2001
From: "Earle F. Philhower, III" <earlephilhower@yahoo.com>
Date: Thu, 26 Jul 2018 09:07:28 -0700
Subject: [PATCH] Fix concat not 0-terminating when String shrunk (#4962)

As @devyte noticed, PR #4955 has an issue when you catenate a string to
itself and the string used to hold a longer value because it does not
explicitly 0-terminate the resulting string.  If the string was extended,
however, reserve() would 0-terminate by default.

Always terminate the result of `s += s;` now.
---
 cores/esp8266/WString.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/cores/esp8266/WString.cpp b/cores/esp8266/WString.cpp
index 9130d1bc1..edc8ab8b5 100644
--- a/cores/esp8266/WString.cpp
+++ b/cores/esp8266/WString.cpp
@@ -270,8 +270,9 @@ unsigned char String::concat(const String &s) {
             return 1;
         if (!reserve(newlen))
             return 0;
-        memcpy(s.buffer + len, s.buffer, len);
+        memcpy(buffer + len, buffer, len);
         len = newlen;
+        buffer[len] = 0;
         return 1;
     } else {
         return concat(s.buffer, s.len);