misc/multistream-select: Replace msg.get(0) with msg.first()

[mxinden]

Description

Fix clippy warning:

error: accessing first element with `msg.get(0)`                                                                                              
   --> misc/multistream-select/src/protocol.rs:182:12                                                                                         
    |                                                                                                                                         
182 |         if msg.get(0) == Some(&b'/')                                                                                                    
    |            ^^^^^^^^^^ help: try: `msg.first()`                                                                                          
    |                                                                                                                                         
    = note: `-D clippy::get-first` implied by `-D warnings`                                                                                   
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first                                 

Links to any relevant issues

Fails CI on #2786

Open Questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates