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.

NEWS

@@ -248,6 +248,10 @@ New in spot 2.4.3.dev (not yet released)
     Fin-less & CNF version of the acceptance condition had several
     unit clauses.
 
+  - If the automaton passed to sbacc() was incomplete because of some
+    unreachable states, then it was possible that the output would
+    marked incomplete while it was in fact complete.
+
 New in spot 2.4.3 (2017-12-19)
 
   Bugs fixed:

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;
   }
 }

tests/core/sbacc.test

@@ -262,3 +262,17 @@ EOF
 
 autfilt --sbacc alt.hoa > out.hoa
 diff out.hoa expect.hoa
+
+# Issue #312
+autfilt -S <<EOF | autfilt --is-complete
+HOA: v1
+States: 2
+Start: 0
+AP: 0
+Acceptance: 1 Inf(0)
+properties: !complete
+--BODY--
+State: 0 [t] 0 {0}
+State: 1
+--END--
+EOF