ArduinoOTA: handle end of packet in readStringUntil

fixes esp8266#3912

Author: yoursunny

libraries/ArduinoOTA/ArduinoOTA.cpp

@@ -159,13 +159,13 @@ int ArduinoOTAClass::parseInt(){
 
 String ArduinoOTAClass::readStringUntil(char end){
   String res = "";
-  char value;
+  int value;
   while(true){
     value = _udp_ota->read();
-    if(value == '\0' || value == end){
+    if(value < 0 || value == '\0' || value == end){
       return res;
     }
-    res += value;
+    res += static_cast<char>(value);
   }
   return res;
 }