avm2: Handle setting tabIndex to -1

Despite throwing an error that `tabIndex` cannot be negative,
the value of -1 is allowed, and it means that `tabIndex` is unset.

core/src/avm2/globals/flash/display/interactive_object.rs

@@ -168,7 +168,9 @@ pub fn set_tab_index<'gc>(
 ) -> Result<Value<'gc>, Error<'gc>> {
     if let Some(obj) = this.as_display_object().and_then(|o| o.as_interactive()) {
         let value = args.get_i32(activation, 0)?;
-        if value < 0 {
+        // Despite throwing an error that tabIndex cannot be negative,
+        // the value of -1 is allowed, and it means that tabIndex is unset.
+        if value < -1 {
             return Err(make_error_2027(activation, value));
         }
         obj.set_tab_index(&mut activation.context, Some(value));