Add virtual destructor to SproutNote and BaseNote

Also add test to make sure default copy constructors are okay.

src/zcash/Note.hpp

@@ -14,6 +14,8 @@ protected:
 public:
     BaseNote() {}
     BaseNote(uint64_t value) : value_(value) {};
+    virtual ~BaseNote() {};
+
     virtual uint256 cm() const {};
     inline uint64_t value() const { return value_; };
 };
@@ -29,6 +31,8 @@ public:
 
     SproutNote();
 
+    virtual ~SproutNote() {};
+
     virtual uint256 cm() const override;
 
     uint256 nullifier(const SpendingKey& a_sk) const;