ddl/ddl.go: Recv from chan `task.err` may block forever when send is skipped

Bug Report

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1. Minimal reproduce step (Required)

We found this bug through a WIP Fuzzing project by system-pclub, PSU

2. What did you expect to see? (Required)

In ddl_worker.go, limitDDLJobs() calls addBatchDDLJobs() on L272 if selecting the first case, which in turn calls sending operation task.err <-err on L311. https://github.com/pingcap/tidb/blob/fa71a0e46d427e8a7450a2dce27f8c0d4c02a45c/ddl/ddl_worker.go#L258-L275 https://github.com/pingcap/tidb/blob/fa71a0e46d427e8a7450a2dce27f8c0d4c02a45c/ddl/ddl_worker.go#L310-L311 Then in ddl.go, doDDLJob() calls receiving operation err := <-task.err on L540 https://github.com/pingcap/tidb/blob/fa71a0e46d427e8a7450a2dce27f8c0d4c02a45c/ddl/ddl.go#L534-L540

3. What did you see instead (Required)

If the select in limitDDLJobs() chooses the second case, the sending operation in function addBatchDDLJobs() will be skipped. Thus the receiving operation in doDDLJob() will block forever.

4. What is your TiDB version? (Required)

branch: master commit: 7e6690df8e8d5474b1872edbd279bb1b3c510ee5