Fix gpdb hooks calling in PortalStart in case of exceptions caught.

[Aegeaner]

This PR is a following fix after #12694, as the queryDesc->estate may be NULL in the catch block, which may cause a PANIC.

According to a reported error in PolicyEagerFreeAssignOperatorMemoryKB makes query end without calling GPDB hooks https://github.com/greenplum-db/gpdb/issues/12690 , the code path in standard_ExecutorStart didn't handle exception in PolicyAutoAssignOperatorMemoryKB and PolicyEagerFreeAssignOperatorMemoryKB calling, which may cause the OOM exception not handled in standard_ExecutorStart but throw to upper PortalStart methods, while there is also exception handling machanism in PortalStart but GPDB hooks will not be called because of portal->queryDesc will be NULL in certain transation states. This PR fixed this issue.

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