Use memmove in String::remove

Before, it used strncpy, but that has undefined behaviour when the
source and destination strings overlap. memove is guaranteed to work
correctly in this case.

Note that memmove simply copies all bytes, whereas strncpy stopped at
the first nul-byte. This measn this commit also makes remove work for
strings that contain embedded nul bytes.

Author: matthijskooijman

hardware/arduino/avr/cores/arduino/WString.cpp

@@ -695,7 +695,7 @@ void String::remove(unsigned int index, unsigned int count){
 	if (index + count > len) { 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;
 }