No StMaker functions if no StMaker loaded (#693)

In ROOT6, it isn't enough to just hide a class's functions inside an
`if`
clause checking the corresponding TClass's existence when the class
isn't
loaded. This can be demonstrated currently in SL24x by simply executing:
```
root4star -b -l -q
```
...which produces an error when executing `rootlogoff.C` at quit. Maybe
there
are other solutions, but this simple proposal breaks `rootlogoff.C` into
two parts,
not even loading the second part unless the first part is true
- test the existence of the StMaker class
- execution of StMaker functions

I welcome other proposals.

StRoot/macros/rootlogoff.C

@@ -1,10 +1,7 @@
 {
   if (TClassTable::GetDict("StMaker"))
   {
-    StMaker* mk = StMaker::GetChain();
-    if (mk) {
-      mk->Finish();
-    }
+    gROOT->Macro("rootlogoff2.C");
   }
 
   std::cout << "\nThis is the end of STAR ROOT -- Goodbye\n" << std::endl;

StRoot/macros/rootlogoff2.C

@@ -0,0 +1,6 @@
+{
+  StMaker* mk = StMaker::GetChain();
+  if (mk) {
+    mk->Finish();
+  }
+}