sbacc: remove spurious initial state in some output

This fixes #492, based on a report from Jérôme Dubois.

* spot/twaalgos/sbacc.cc: If the initial state is in a rejecting
component, start with an initial state whose colors are unsat_mark.
* tests/core/sbacc.test: Add test case.
* tests/python/pdegen.py: Adjust it.

spot/twaalgos/sbacc.cc

@@ -145,6 +145,8 @@ namespace spot
           // Use any edge going into the initial state to set the first
           // acceptance mark.
           init_acc = one_in[s] | common_out[s];
+        else
+          init_acc = unsat_mark.second;
 
         old_st.push_back(new_state(s, init_acc));
       }