FunctionPointerBase::call() no check for valid if fptr has been set

FunctionPointerBase method call()

```
// taken from FunctionPointerBase.h
inline R call(void* arg) {
return _membercaller(_object, _member, arg);
}
```

does not check if _membercaller or _object are valid.

Start with FunctionPointerBase.h and the FunctionPointerBase::call() implementation shown in the issue. Determine the project's expected behavior when _membercaller or _object is invalid, then add coverage for those states. Done means call() handles invalid targets safely and the relevant tests pass.

Written by the indexing model from the issue text.