fix: don't adjust parent tree's stack size when composting

src/bin/ponderosa-cpu.rs

@@ -234,7 +234,14 @@ fn main() -> io::Result<()> {
                 let tree = trees.remove(&tree_id).unwrap();
                 (tree.parent_id, tree.stack_size)
             };
-            let parent = trees.get_mut(&parent_id).unwrap();
+
+            // One tree is "its own" parent, namely, the seed tree, in which
+            // case this `get_mut` will return None
+            let parent = match trees.get_mut(&parent_id) {
+                Some(p) => p,
+                None => continue
+            };
+
             parent.stack_size += recovered_stack_size;
         }