Improve public_key::decode_account to not write result on error (#3794)

public_key::decode_account() would overwrite the object it was called
on error, if the error was with the checksum part of the address.

This commit changes the function to work on a temporary object and
store the result only on success.

resolves  #3793

nano/core_test/uint256_union.cpp

@@ -375,6 +375,20 @@ TEST (uint256_union, decode_nano_variant)
 	ASSERT_FALSE (key.decode_account ("nano_1111111111111111111111111111111111111111111111111111hifc8npp"));
 }
 
+/**
+ * It used to be the case that when the address was wrong only in the checksum part
+ * then the decode_account would return error and it would also write the address with
+ * fixed checksum into 'key', which is not desirable.
+ */
+TEST (uint256_union, key_is_not_updated_on_checksum_error)
+{
+	nano::account key;
+	ASSERT_EQ (key, 0);
+	bool result = key.decode_account ("nano_3e3j5tkog48pnny9dmfzj1r16pg8t1e76dz5tmac6iq689wyjfpiij4txtd1");
+	ASSERT_EQ (key, 0);
+	ASSERT_TRUE (result);
+}
+
 TEST (uint256_union, account_transcode)
 {
 	nano::account value;

nano/lib/numbers.cpp

@@ -116,14 +116,18 @@ bool nano::public_key::decode_account (std::string const & source_a)
 					}
 					if (!error)
 					{
-						*this = (number_l >> 40).convert_to<nano::uint256_t> ();
+						nano::public_key temp = (number_l >> 40).convert_to<nano::uint256_t> ();
 						uint64_t check (number_l & static_cast<uint64_t> (0xffffffffff));
 						uint64_t validation (0);
 						blake2b_state hash;
 						blake2b_init (&hash, 5);
-						blake2b_update (&hash, bytes.data (), bytes.size ());
+						blake2b_update (&hash, temp.bytes.data (), temp.bytes.size ());
 						blake2b_final (&hash, reinterpret_cast<uint8_t *> (&validation), 5);
 						error = check != validation;
+						if (!error)
+						{
+							*this = temp;
+						}
 					}
 				}
 				else