Fix incorrect count of fields passed to tuple deserialization methods

This count should mean the number of fields expected in the serialized form,
so if some fields are skipped, they shouldn't be counted

Methods affected:
- Deserializer::deserialize_tuple
- Deserializer::deserialize_tuple_struct
- VariantAccess::tuple_variant

test_suite/tests/test_annotations.rs

@@ -1533,7 +1533,7 @@ fn test_invalid_length_enum() {
             Token::TupleVariant {
                 name: "InvalidLengthEnum",
                 variant: "B",
-                len: 3,
+                len: 2,
             },
             Token::I32(1),
             Token::TupleVariantEnd,