chunking: Handle low amount of high-size packages

If we have a low amount of high-size packages then we will drain with an
index higher than the size of the Vec which triggers a panic.

Fixes: coreos/rpm-ostree#4646

lib/src/chunking.rs

@@ -462,8 +462,12 @@ fn get_partitions_with_threshold<'a>(
     }
 
     // Extra high-size packages
-    let mut remaining_pkgs: Vec<_> = high_size.drain(limit_hs_bins..).collect();
-    assert_eq!(high_size.len(), limit_hs_bins);
+    let mut remaining_pkgs: Vec<_> = if high_size.len() < limit_hs_bins {
+        Vec::new()
+    } else {
+        high_size.drain(limit_hs_bins..).collect()
+    };
+    assert_lt!(high_size.len(), limit_hs_bins);
 
     // Concatenate extra high-size packages + med_sizes to keep it descending sorted
     remaining_pkgs.append(&mut med_size);