wallet2: reject zero keys in json input

Because the user might do this for reasons unknown.
Values beyond l-1 will be reduced, so are accepted.

Reported by who-biz.

src/wallet/wallet2.cpp

@@ -591,6 +591,8 @@ std::pair<std::unique_ptr<tools::wallet2>, tools::password_container> generate_f
       }
       viewkey = *reinterpret_cast<const crypto::secret_key*>(viewkey_data.data());
       crypto::public_key pkey;
+      if (viewkey == crypto::null_skey)
+        THROW_WALLET_EXCEPTION(tools::error::wallet_internal_error, tools::wallet2::tr("view secret key may not be all zeroes"));
       if (!crypto::secret_key_to_public_key(viewkey, pkey)) {
         THROW_WALLET_EXCEPTION(tools::error::wallet_internal_error, tools::wallet2::tr("failed to verify view key secret key"));
       }
@@ -607,6 +609,8 @@ std::pair<std::unique_ptr<tools::wallet2>, tools::password_container> generate_f
       }
       spendkey = *reinterpret_cast<const crypto::secret_key*>(spendkey_data.data());
       crypto::public_key pkey;
+      if (spendkey == crypto::null_skey)
+        THROW_WALLET_EXCEPTION(tools::error::wallet_internal_error, tools::wallet2::tr("spend secret key may not be all zeroes"));
       if (!crypto::secret_key_to_public_key(spendkey, pkey)) {
         THROW_WALLET_EXCEPTION(tools::error::wallet_internal_error, tools::wallet2::tr("failed to verify spend key secret key"));
       }