sbacc: fix sbacc producing complete automata marked as incomplete

Fixes #312, reported by František Blahoudek.

* spot/twaalgos/sbacc.cc: Detect the case were this can happen,
and fix it.
* tests/core/sbacc.test: New test case.
* NEWS: Mention the bug.

spot/twaalgos/sbacc.cc

@@ -168,6 +168,17 @@ namespace spot
           }
       }
     res->merge_edges();
+
+    // If the automaton was marked as not complete, and we have
+    // ignored some unreachable state, then it is possible that the
+    // result becomes complete.
+    if (res->prop_complete().is_false())
+      for (unsigned i = 0; i < ns; ++i)
+        if (!si.reachable_state(i))
+          {
+            res->prop_complete(trival::maybe());
+            break;
+          }
     return res;
   }
 }