rust-dirstatemap: use a checked sub instead of a potentially underflowing one

rust-dirstatemap: use a checked sub instead of a potentially underflowing one

This was missed in 2593873cda0f

Differential Revision: https://phab.mercurial-scm.org/D12532

--- a/rust/hg-core/src/dirstate_tree/dirstate_map.rs	Fri Apr 08 17:55:03 2022 +0200
+++ b/rust/hg-core/src/dirstate_tree/dirstate_map.rs	Fri Apr 08 18:10:12 2022 +0200
@@ -1337,7 +1337,9 @@
             )?
             .and_then(|node| {
                 if let Some(source) = &node.copy_source {
-                    *count -= 1;
+                    *count = count
+                        .checked_sub(1)
+                        .expect("nodes_with_copy_source_count should be >= 0");
                     DirstateMap::count_dropped_path(unreachable_bytes, source);
                 }
                 node.copy_source.take().map(Cow::into_owned)