Don't mess with the original in String::substring

From upstream ArduinoCore-API commit dd236bfd2c62ed083aaa1b1229d71e85fd92f7a9

Author: rcombs

cores/esp32/WString.cpp

@@ -711,10 +711,7 @@ String String::substring(unsigned int left, unsigned int right) const {
         return out;
     if(right > len())
         right = len();
-    char temp = buffer()[right];  // save the replaced character
-    wbuffer()[right] = '\0';
-    out = wbuffer() + left;  // pointer arithmetic
-    wbuffer()[right] = temp;  //restore character
+    out.copy(buffer() + left, right - left);
     return out;
 }