MDEV-18105 Mariabackup fails to copy encrypted InnoDB system tablespace if LSN>4G

This is a regression caused by
commit 8c43f963
that was part of the MDEV-12112 fixes.

page_is_corrupted(): Never interpret page_no=0 as encrypted.

extra/mariabackup/fil_cur.cc

@@ -306,9 +306,16 @@ static bool page_is_corrupted(byte *page, ulint page_no, xb_fil_cur_t *cursor, f
 		return false;
 	}
 
-	/* Validate encrypted pages. */
-	if (mach_read_from_4(page + FIL_PAGE_FILE_FLUSH_LSN_OR_KEY_VERSION) && 
-	(space->crypt_data && space->crypt_data->type!= CRYPT_SCHEME_UNENCRYPTED)) {
+	/* Validate encrypted pages. The first page is never encrypted.
+	In the system tablespace, the first page would be written with
+	FIL_PAGE_FILE_FLUSH_LSN at shutdown, and if the LSN exceeds
+	4,294,967,295, the mach_read_from_4() below would wrongly
+	interpret the page as encrypted. We prevent that by checking
+	page_no first. */
+	if (page_no
+	    && mach_read_from_4(page + FIL_PAGE_FILE_FLUSH_LSN_OR_KEY_VERSION)
+	    && space->crypt_data
+	    && space->crypt_data->type != CRYPT_SCHEME_UNENCRYPTED) {
 
 		if (!fil_space_verify_crypt_checksum(page, cursor->zip_size))
 			return true;

mysql-test/suite/mariabackup/huge_lsn.opt

 --innodb-encrypt-log=ON
+--innodb-tablespaces-encryption
+--innodb-encrypt-tables=ON
+--innodb-encryption-rotate-key-age=1
+--innodb-encryption-threads=4
 --plugin-load-add=$FILE_KEY_MANAGEMENT_SO
 --loose-file-key-management
 --loose-file-key-management-filekey=FILE:$MTR_SUITE_DIR/filekeys-data.key