signal.rs: crash a bit earlier when signal number is negative

The conversion to usize is used for array accesses, so negative values
would cause crashes either way. Let's do it earlier so we can get rid of
the suspect C-style cast.

fish-rust/src/signal.rs

@@ -323,7 +323,7 @@ impl From<Signal> for i32 {
 
 impl From<Signal> for usize {
     fn from(value: Signal) -> Self {
-        value.code() as usize
+        usize::try_from(value.code()).unwrap()
     }
 }