fix: panic on completion for an alias (#12090)

crates/nu-cli/src/completions/command_completions.rs

@@ -186,6 +186,9 @@ impl Completer for CommandCompletion {
 
         // The last item here would be the earliest shape that could possible by part of this subcommand
         let subcommands = if let Some(last) = last {
+            // we'll check if start of span is greater than offset, to prevent panic
+            // else we can use the start and end values from last and use that.
+            if last.0.start >= offset {
                 self.complete_commands(
                     working_set,
                     Span::new(last.0.start, pos),
@@ -193,6 +196,15 @@ impl Completer for CommandCompletion {
                     false,
                     options.match_algorithm,
                 )
+            } else {
+                self.complete_commands(
+                    working_set,
+                    Span::new(last.0.start, last.0.end),
+                    offset,
+                    false,
+                    options.match_algorithm,
+                )
+            }
         } else {
             vec![]
         };