Fix 'attempt to divide by zero' panic

[nuid64]

Helix crashed when I cut the window too much.

thread 'main' panicked at 'attempt to divide by zero', helix-term/src/ui/text.rs:61:23
stack backtrace:
   0: rust_begin_unwind
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:64:14
   2: core::panicking::panic
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:111:5
   3: helix_term::ui::text::required_size
   4: <helix_term::ui::markdown::Markdown as helix_term::compositor::Component>::required_size
   5: <helix_term::ui::popup::Popup<T> as helix_term::compositor::Component>::required_size
   6: <helix_term::ui::popup::Popup<T> as helix_term::compositor::Component>::render
   7: helix_term::application::Application::render::{{closure}}
   8: helix_term::application::Application::run::{{closure}}
   9: tokio::runtime::park::CachedParkThread::block_on
  10: tokio::runtime::runtime::Runtime::block_on
  11: hx::main

[archseer]

You want to use saturating_div instead which avoids the unwrap

[archseer]

Ah I guess that doesn't check for 0

[nuid64]

Also I found two required_size functions in helix-term/src/ui/text.rs and one of them returns (u16, u16) not wrapped in Option. It's confusing to have two of them, both of which perform actions on the text.