Avoid reading past end of non-zero terminated char arrays

cores/esp8266/WString.cpp

@@ -260,7 +260,8 @@ String &String::copy(const char *cstr, unsigned int length) {
  return *this;
  }
  setLen(length);
- memmove_P(wbuffer(), cstr, length + 1);
+ memmove_P(wbuffer(), cstr, length);
+ wbuffer()[length] = 0;
  return *this;
 }
 
@@ -270,7 +271,8 @@ String &String::copy(const __FlashStringHelper *pstr, unsigned int length) {
  return *this;
  }
  setLen(length);
- memcpy_P(wbuffer(), (PGM_P)pstr, length + 1); // We know wbuffer() cannot ever be in PROGMEM, so memcpy safe here
+ memcpy_P(wbuffer(), (PGM_P)pstr, length); // We know wbuffer() cannot ever be in PROGMEM, so memcpy safe here
+ wbuffer()[length] = 0;
  return *this;
 }
 
@@ -411,8 +413,9 @@ bool String::concat(const __FlashStringHelper *str) {
  unsigned int newlen = len() + length;
  if (!reserve(newlen))
  return false;
- memcpy_P(wbuffer() + len(), (PGM_P)str, length + 1);
+ memcpy_P(wbuffer() + len(), (PGM_P)str, length);
  setLen(newlen);
+ wbuffer()[newlen] = 0;
  return true;
 }