Tonic's `encode_item` function is not exception safe, leading to use of uninitialized memory

## Bug Report

### Version

0.14.6

### Platform

Linux

### Crates

`tonic`

### Description

Hello, we are security researchers targeting Rust security. By running our tool in this repository, we found a soundness issue. **This report is written by 100% human**. We promise that all you read will never be generated by LLM.

The problem happens here:

https://github.com/grpc/grpc-rust/blob/b68df9ff58eb8e4fc402ba2f0c3f6f56e1e5ead0/tonic/src/codec/encode.rs#L133-L179

In line 149, `buf.advance_mut` is called to increase the length for header, and in line 178, `finish_encoding` is called to fill header bytes into the increased slots. However, in line 156 and line 173, `Encoder::encode` method is called, which is a user-provided trait method. If user deliberately call `panic!` inside `encode` function, `finish_encoding` will never be called, leading to `buf` exposing uninitialized variable.

To fix it, I think adding a drop guard to restore the length when unwinding could help.