[Bug] Fix count_inner_msg_num method of message decoder mode #520

Before Creating the Bug Report

• I found a bug, not just asking a question, which should be created in GitHub Discussions.

• I have searched the GitHub Issues and GitHub Discussions of this repository and believe that this is not a duplicate.

• I have confirmed that this bug belongs to the current repository, not other repositories of RocketMQ.

Runtime platform environment

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RocketMQ Rust version

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Rust Version

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Describe the Bug

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Steps to Reproduce

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What Did You Expect to See?

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What Did You See Instead?

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Additional Context

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