Add some missing viewkey methods that cause compile errors, unrelated but necessary

src/zcash/Address.cpp

@@ -37,3 +37,15 @@ PaymentAddress SpendingKey::address() const {
 }
 
 }
+
+bool IsValidPaymentAddress(const libzcash::PaymentAddress& zaddr) {
+    return zaddr.which() != 0;
+}
+
+bool IsValidViewingKey(const libzcash::ViewingKey& vk) {
+    return vk.which() != 0;
+}
+
+bool IsValidSpendingKey(const libzcash::SpendingKey& zkey) {
+    return zkey.which() != 0;
+}

src/zcash/Address.hpp

@@ -88,4 +88,13 @@ public:
 
 }
 
+/** Check whether a PaymentAddress is not an InvalidEncoding. */
+bool IsValidPaymentAddress(const libzcash::PaymentAddress& zaddr);
+
+/** Check whether a ViewingKey is not an InvalidEncoding. */
+bool IsValidViewingKey(const libzcash::ViewingKey& vk);
+
+/** Check whether a SpendingKey is not an InvalidEncoding. */
+bool IsValidSpendingKey(const libzcash::SpendingKey& zkey);
+
 #endif // ZC_ADDRESS_H_