Panic in SchemaStore

[hongyunyan]

What did you do?

run test for ail_over_ddl_mix with the pr https://github.com/pingcap/ticdc/pull/961/files

What did you expect to see?

pass the test

What did you see instead?

the log is in of /tmp/tidb_cdc_test/fail_over_ddl_mix_backup 10.2.6.126

Versions of the cluster

Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

(paste TiDB cluster version here)

Upstream TiKV version (execute tikv-server --version):

(paste TiKV version here)

TiCDC version (execute cdc version):

(paste TiCDC version here)