MDEV-23982: Mariabackup hangs on backup

MDEV-13318 introduced a condition to Mariabackup that can cause it to
hang if the server goes idle after writing a log block that has no
payload after the 12-byte header. Normal recovery in log0recv.cc would
allow blocks with exactly 12 bytes of length, and only reject blocks
where the length field is shorter than that.

extra/mariabackup/xtrabackup.cc

@@ -2723,7 +2723,7 @@ static lsn_t xtrabackup_copy_log(lsn_t start_lsn, lsn_t end_lsn, bool last)
 			scanned_lsn += data_len;
 		} else if (data_len
 			   >= OS_FILE_LOG_BLOCK_SIZE - LOG_BLOCK_TRL_SIZE
-			   || data_len <= LOG_BLOCK_HDR_SIZE) {
+			   || data_len < LOG_BLOCK_HDR_SIZE) {
 			/* We got a garbage block (abrupt end of the log). */
 			msg(0,"garbage block: " LSN_PF ",%zu",scanned_lsn, data_len);
 			break;