Can call `get_succesor()` method of `fsm` class without setting a successor.

[kingtom22]

Hi,

Firstly, thank You for developing and maintaining this library, it is very useful.
I found that get_succesor() method of fsm can be called without setting a successor and the function will dereference a nullptr in that case. It is not written in the documentation that the user of the framework must call the has_succesor() method before using the get_succesor() method. I recommend to throw an exception or add this requirement to the documentation.

[jwellbelove]

I shall look at adding an ETL_ASSERT

[jwellbelove]

It may also be viable to make the return an etl::optional.

[jwellbelove]

I've just checked, and get_successor() asserts an etl::successor_invalid if no successor has been set.

[kingtom22]

I see, in version 19.5.0 etl::succesor has been added, and that implements that check before using pointer of the successor. The issue can be closed.
Thank You, for Your help!