A block in a tar file is 512 bytes. Period.

Remove the entirely pointless symbolic constant
and sizeof(unsigned char).  (The constant
here is doubly wrong, since not only does
it obscure a basic format constant, it was
never intended to be a tar-specific value,
so could conceivably be changed at some point
in the future.)

lib/libarchive/archive_read_support_format_tar.c

@@ -714,7 +714,7 @@ archive_block_is_null(const unsigned char *p)
 {
 	unsigned i;
 
-	for (i = 0; i < ARCHIVE_BYTES_PER_RECORD / sizeof(*p); i++)
+	for (i = 0; i < 512; i++)
 		if (*p++)
 			return (0);
 	return (1);