[repacker] fix fuzzer timeout.

For https://oss-fuzz.com/testcase-detail/5845846876356608. Only process the set of unique overflows.

src/graph/serialize.hh

@@ -33,6 +33,23 @@ struct overflow_record_t
 {
   unsigned parent;
   unsigned child;
+
+  bool operator != (const overflow_record_t o) const
+  { return !(*this == o); }
+
+  inline bool operator == (const overflow_record_t& o) const
+  {
+    return parent == o.parent &&
+        child == o.child;
+  }
+
+  inline uint32_t hash () const
+  {
+    uint32_t current = 0;
+    current = current * 31 + hb_hash (parent);
+    current = current * 31 + hb_hash (child);
+    return current;
+  }
 };
 
 inline
@@ -94,6 +111,7 @@ will_overflow (graph_t& graph,
   if (overflows) overflows->resize (0);
   graph.update_positions ();
 
+  hb_hashmap_t<overflow_record_t*, bool> record_set;
   const auto& vertices = graph.vertices_;
   for (int parent_idx = vertices.length - 1; parent_idx >= 0; parent_idx--)
   {
@@ -109,7 +127,10 @@ will_overflow (graph_t& graph,
       overflow_record_t r;
       r.parent = parent_idx;
       r.child = link.objidx;
+      if (record_set.has(&r)) continue; // don't keep duplicate overflows.
+
       overflows->push (r);
+      record_set.set(&r, true);
     }
   }