fix webif xml encoding, part 2

Origin commit data
------------------
Commit: 6b449f5d95
Author: TangoCash <eric@loxat.de>
Date: 2018-03-28 (Wed, 28 Mar 2018)

src/system/helpers.cpp

@@ -1651,3 +1651,60 @@ bool parseJsonFromString(string& jData, Json::Value *root, string *errMsg)
 	delete reader;
 	return ret;
 }
+
+std::string iso_8859_1_to_utf8(std::string &str)
+{
+	std::string strOut;
+	for (std::string::iterator it = str.begin(); it != str.end(); ++it)
+	{
+		uint8_t ch = *it;
+		if (ch < 0x80)
+		{
+			strOut.push_back(ch);
+		}
+		else
+		{
+			strOut.push_back(0xc0 | ch >> 6);
+			strOut.push_back(0x80 | (ch & 0x3f));
+		}
+	}
+	return strOut;
+}
+
+bool utf8_check_is_valid(const std::string &str)
+{
+	int c, i, ix, n, j;
+	for (i = 0, ix = str.length(); i < ix; i++)
+	{
+		c = (unsigned char) str[i];
+		/*
+		if (c==0x09 || c==0x0a || c==0x0d || (0x20 <= c && c <= 0x7e))
+			n = 0; // is_printable_ascii
+		*/
+		if (0x00 <= c && c <= 0x7f)
+			n = 0; // 0bbbbbbb
+		else if ((c & 0xE0) == 0xC0)
+			n = 1; // 110bbbbb
+		else if (c == 0xed && i < (ix - 1) && ((unsigned char)str[i + 1] & 0xa0) == 0xa0)
+			return false; // U+d800 to U+dfff
+		else if ((c & 0xF0) == 0xE0)
+			n = 2; // 1110bbbb
+		else if ((c & 0xF8) == 0xF0)
+			n = 3; // 11110bbb
+		/*
+		else if (($c & 0xFC) == 0xF8)
+			n=4; // 111110bb //byte 5, unnecessary in 4 byte UTF-8
+		else if (($c & 0xFE) == 0xFC)
+			n=5; // 1111110b //byte 6, unnecessary in 4 byte UTF-8
+		*/
+		else
+			return false;
+
+		for (j = 0; j < n && i < ix; j++) // n bytes matching 10bbbbbb follow?
+		{
+			if ((++i == ix) || (((unsigned char)str[i] & 0xC0) != 0x80))
+				return false;
+		}
+	}
+	return true;
+}