Auto merge of #3390 - arcalinea:3319_discard_sk_if_invalid_ivk, r=str4d

Discard sk if ivk == 0

Addresses #3319

src/zcash/Address.hpp

@@ -153,6 +153,7 @@ public:
     }
 
     SaplingIncomingViewingKey in_viewing_key() const;
+    bool is_valid() const;
 
     friend inline bool operator==(const SaplingFullViewingKey& a, const SaplingFullViewingKey& b) {
         return a.ak == b.ak && a.nk == b.nk && a.ovk == b.ovk;