fix(esp32): Fix appending to Strings longer than 64k (espressif#11523)

If oldLen is truncated to uint16_t, appending to a String
that is longer than 65535 bytes will create a broken string.

Author: MattiasTF

cores/esp32/WString.cpp

@@ -180,15 +180,15 @@ bool String::changeBuffer(unsigned int maxStrLen) {
   if (maxStrLen < sizeof(sso.buff) - 1) {
     if (isSSO() || !buffer()) {
       // Already using SSO, nothing to do
-      uint16_t oldLen = len();
+      size_t oldLen = len();
       setSSO(true);
       setLen(oldLen);
     } else {  // if bufptr && !isSSO()
       // Using bufptr, need to shrink into sso.buff
       char temp[sizeof(sso.buff)];
       memcpy(temp, buffer(), maxStrLen);
       free(wbuffer());
-      uint16_t oldLen = len();
+      size_t oldLen = len();
       setSSO(true);
       memcpy(wbuffer(), temp, maxStrLen);
       setLen(oldLen);
@@ -201,7 +201,7 @@ bool String::changeBuffer(unsigned int maxStrLen) {
   if (newSize > CAPACITY_MAX) {
     return false;
   }
-  uint16_t oldLen = len();
+  size_t oldLen = len();
   char *newbuffer = (char *)realloc(isSSO() ? nullptr : wbuffer(), newSize);
   if (newbuffer) {
     size_t oldSize = capacity() + 1;  // include NULL.