fix bug

crates/polars-core/src/frame/group_by/perfect.rs

@@ -1,4 +1,5 @@
 use std::fmt::Debug;
+use std::mem::MaybeUninit;
 
 use num_traits::{FromPrimitive, ToPrimitive};
 use polars_utils::idx_vec::IdxVec;
@@ -113,13 +114,14 @@ where
         } else {
             let mut groups = Vec::with_capacity(len);
             let mut first = Vec::with_capacity(len);
+            let first_out = first.spare_capacity_mut();
             groups.resize_with(len, || IdxVec::with_capacity(group_capacity));
 
             let mut push_to_group = |cat, row_nr| unsafe {
                 let buf: &mut IdxVec = groups.get_unchecked_release_mut(cat);
                 buf.push(row_nr);
                 if buf.len() == 1 {
-                    *first.get_unchecked_release_mut(cat) = row_nr;
+                    *first_out.get_unchecked_release_mut(cat) = MaybeUninit::new(row_nr);
                 }
             };