rar: Fix OOB access with unicode filenames

[stoeckmann]

Prevent out of boundary accesses by revalidating offset every time it is incremented.

[stoeckmann]

Again it takes a specialized proof of concept binary to allow address sanitizer to detect this OOB access.

Proof of Concept:

1. Compile libarchive with -fsanitize=address
2. Create poc.rar

base64 -d <<< UmFyIRoHAAZBdAACIQAAAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAAAAAAAAAAAAAA= > poc.rar

3. Create proof of concept code (write into poc.c)

#include <archive.h>
#include <err.h>

int
main(void)
{
	struct archive *a;
	struct archive_entry *entry;
	int r;

	a = archive_read_new();
	archive_read_support_format_rar(a);
	
	if (archive_read_open_filename(a, NULL, 26))
		errx(1, "archive_read_open_filename");
	if (archive_read_next_header(a, &entry))
		errx(2, "archive_read_next_header");

	archive_read_close(a);
	archive_read_free(a);

	return 0;
}

4. Compile poc.c

cc -fsanitize=address -larchive -o poc poc.c

5. Run poc with pipe input (to avoid disk-optimized block sizes)

cat poc.rar | ./poc     # maybe set LD_LIBRARY_PATH to ASAN'ed library