Don't use iterator addresses in IteratorComparator

The addresses of the iterator values are non-deterministic (i.e. they
depend on where the values were allocated). This causes stability issues
when fuzzing (e.g. in the `txorphan` and `mini_miner` harnesses), due
the orders (derived from IteratorComparator) not being deterministic.

Improve stability by comparing the first element in the iterator value
pair instead of using the the value addresses.

src/node/mini_miner.h

@@ -63,7 +63,7 @@ struct IteratorComparator
     template<typename I>
     bool operator()(const I& a, const I& b) const
     {
-        return &(*a) < &(*b);
+        return a->first < b->first;
     }
 };
 

src/txorphanage.h

@@ -92,7 +92,7 @@ protected:
         template<typename I>
         bool operator()(const I& a, const I& b) const
         {
-            return &(*a) < &(*b);
+            return a->first < b->first;
         }
     };