Fix issue with indirect return for C++ member function w/no args

[myronahn]

A FFI C++ member function (i.e. function w/receiver) with no args and an indirect return (i.e. one that returns a Result<T>) was generating a function signature with a missing comma between the self arg and the indirect return parameter.

The get_fail() test member function will trigger this error.

The one-liner fix is in gen/src/write.rs

[dtolnay]

Thanks, published in 0.3.4.