#88 Add check against nullptr returned from fmi2GetString() calls

[Andreas Nicolai]

This is a fix for broken FMUs that return fmi2Ok from getString, yet do not modify the passed string. Since current implementation relies on valid strings to be returned, not doing so (bad FMU) crashes MasterSim.

Suggested solution: initialize pointer to string with nullptr and check if FMU did indeed set a pointer.
Also check with FMI group if there should be a remark about this in the standard.