[Bug]: querycoord panic when generating plans in multi target balancer

[sunby]

Is there an existing issue for this?

• [X] I have searched the existing issues

Environment

- Milvus version: master/2.4
- Deployment mode(standalone or cluster):
- MQ type(rocksmq, pulsar or kafka):    
- SDK version(e.g. pymilvus v2.0.0rc2):
- OS(Ubuntu or CentOS): 
- CPU/Memory: 
- GPU: 
- Others:

Current Behavior

balancer does not check segment number before generate random number.

Expected Behavior

No response

Steps To Reproduce

No response

Milvus Log

No response

Anything else?

No response

[yanliang567]

/assign @sunby /unassign

[stale[bot]]

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