In the Arduino IDE, why does the second call of a function change the previous call results? - function

Question

I tried to write a function that converts a string of hex values into a string of Unicode UTF-8 characters. When this function is called once, everything is fine. But when the function is called twice in succession with the same or different arguments, both output strings are meaningless.
void HEX2String(String* HEXstr, String* str) {
String s2 = "", s3 = "";
long c, c1, c0;
char ch[2] = { 0 };
for (int i = 0; i <= HEXstr->length() - 4; i = i + 4) {
s2 = HEXstr->substring(i, i + 1) + "x" + HEXstr->substring(i + 1, i + 4);
c = (hex2long(&s2));
if (c < 255)
*str += String((char)c);
else {
c1 = (128 + (c & B111111));
c0 = (192 + (c >> 6));
ch[1] = c1;
ch[0] = c0;
str->concat(ch);
}
}
}
String str1 = "0628064700200646062706450020062E062F0627000A0633064406270645000A064806310648062F0020062806470020063306CC0633062A06450020062A064806330637";
String str = "0628064700200646062706450020062E062F0627000A0633064406270645000A064806310648062F0020062806470020063306CC0633062A06450020062A064806330637000A00730061006C0061006D0020006200610072002000730068006F006D0061";
String msg = "";
void setup() {
Serial.begin(9600);
//First call
HEX2String(&str, &msg);
Serial.println(msg);
msg = "";
//Second call
HEX2String(&str1, &msg);
Serial.println(msg);
}
void main() {
//
}
If I comment the second call, the output in the serial monitor is:
سلام
ورود به سیستم توسط
salam bar shoma
It is correct. If the second call is not commented, the output in the serial monitor is:
ب⸮⸮ه⸮⸮ ن⸮⸮ا⸮⸮م⸮⸮ خ⸮⸮د⸮⸮ا⸮⸮
س⸮⸮ل⸮⸮ا⸮⸮م⸮⸮
و⸮⸮ر⸮⸮و⸮⸮د⸮⸮ ب⸮⸮ه⸮⸮ س⸮⸮ی⸮⸮س⸮⸮ت⸮⸮م⸮⸮ ت⸮⸮و⸮⸮س⸮⸮ط⸮⸮
salam bar shomaب⸮⸮ه⸮⸮ ن⸮⸮ا⸮⸮م⸮⸮ خ⸮⸮د⸮⸮ا⸮⸮
س⸮⸮ل⸮⸮ا⸮⸮م⸮⸮
و⸮⸮ر⸮⸮و⸮⸮د⸮⸮ ب⸮⸮ه⸮⸮ س⸮⸮ی⸮⸮س⸮⸮ت⸮⸮م⸮⸮ ت⸮⸮و⸮⸮س⸮⸮ط⸮⸮

Answer 1

C-strings need to be null terminated. Your ch is not.
Define it as 3 characters:
char ch[3] = { 0 };
and add a null terminator:
ch[0] = c0;
ch[1] = c1;
ch[2] = 0;