fix panic in subtract (vercel/turborepo#7859)

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Closes PACK-2866

crates/turbo-tasks-memory/src/count_hash_set.rs

@@ -301,7 +301,7 @@ impl<'a, T> Iterator for CountHashSetIter<'a, T> {
     type Item = &'a T;
 
     fn next(&mut self) -> Option<Self::Item> {
-        self.count -= 1;
+        self.count = self.count.saturating_sub(1);
         self.inner.next()
     }