Crash when catting random bytes

[raphCode]

While filing #1345, I reliably got some crashes on zellij v0.28.1.
Do a cat /dev/urandom inside zellij, after some seconds to minutes I hit:

Error occurred in server:

  × Thread 'screen' panicked.
  ├─▶ Originating Thread(s)
  │   	1. screen_thread: HandlePtyBytes
  │
  ├─▶ At zellij-server/src/panes/grid.rs:1543:46
  ╰─▶ index out of bounds: the len is 1 but the index is 1

[imsnif]

Yeah, both this and #1345 are regressions unfortunately. I used to do exactly these sorts of tests in the early days :)
I'll do another round this week to patch things up and hopefully soon we'll add some proper error handling there so that even if this happens, at worst the specific terminal pane will glitch a little.