Fix abnormal termination caused by use of an invalid context when AlgExecStateSvc is verbose

The crash is caused by !248 (merged), namely, by the wrong assumption used in AlgExecStateSvc::dump(..) that an event context is always valid. And in AlgExecStateSvc::init() it never is.

The undefined behavior, occurring if AlgExecStateSvc is put to VERBOSE, makes Gaudi crash throwing an std::out_of_range upon the use of an uninitialized slot ID.

Not sure how much useful the dump in AlgExecStateSvc::init() is.. But, in case it is, I preferred checking the context validity to removing the entire dump, thus resurrecting the behavior we had before the above mentioned MR.