MDEV-25815 mariabackup crash or debug assert with --backup --databases-exclude

Fix regression (debug assertion or division by 0)
caused by cfd3d70c

extra/mariabackup/xtrabackup.cc

@@ -4048,7 +4048,7 @@ xb_register_filter_entry(
 		strncpy(dbname, name, p - name);
 		dbname[p - name] = 0;
 
-		if (databases_hash) {
+		if (databases_hash && databases_hash->array) {
 			HASH_SEARCH(name_hash, databases_hash,
 					ut_fold_string(dbname),
 					xb_filter_entry_t*,

mysql-test/suite/mariabackup/xbstream.test

@@ -8,7 +8,7 @@ mkdir $targetdir;
 let $streamfile=$MYSQLTEST_VARDIR/tmp/backup.xb;
 
 echo # xtrabackup backup to stream;
-exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup  --stream=xbstream > $streamfile 2>$targetdir/backup_stream.log;
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup --databases-exclude=foobar --stream=xbstream > $streamfile 2>$targetdir/backup_stream.log;
 echo # xbstream extract;
 --disable_result_log
 exec $XBSTREAM -x -C $targetdir  < $streamfile;