Callback member initialisation.

[davidhjp01]

I found that stepFinished is not initialised here and it may be initialised to a garbage number (non-zero) depending on the compiler/platform etc. It seems some of the fmus may check if the function is initialised (if it is zero or not) and calling it resulted in crashing proxyfmu.

This patch just adds nullptr to stepFinished to avoid this issue.

[markaren]

Thanks! Without checking, does FMI1 handling have a similar issue?

[davidhjp01]

I think fmi1_slave may also have the same issue. I just added initialisation there too to make sure the callers can check if the stepFinished is initialised.