Improve value return on err

With #18 a compile error was fixed, but not in the most optimal way, because it produces an unnecessary allocation in the error case.

https://github.com/jmattheis/goverter/blob/5dba9cdc1fa7571c261721ad5a95649657883f27/scenario/3_struct_extend_pattern.yml#L45-L55

Currently, goverter doesn't know if there is already a variable existing with the right type, maybe there is a smart way to do this?