Fix undefined behaviour for overlapping String

Port of arduino/ArduinoCore-avr#20 , all credits goes to @earlephilhower

Author: facchinm

Diff for: api/String.cpp

@@ -698,7 +698,7 @@ void String::remove(unsigned int index, unsigned int count){
 	if (count > len - index) { count = len - index; }
 	char *writeTo = buffer + index;
 	len = len - count;
-	strncpy(writeTo, buffer + index + count,len - index);
+	memmove(writeTo, buffer + index + count,len - index);
 	buffer[len] = 0;
 }
 
@@ -726,7 +726,7 @@ void String::trim(void)
 	char *end = buffer + len - 1;
 	while (isspace(*end) && end >= begin) end--;
 	len = end + 1 - begin;
-	if (begin > buffer) memcpy(buffer, begin, len);
+	if (begin > buffer) memmove(buffer, begin, len);
 	buffer[len] = 0;
 }