Fix "BDD Error" in scc_filter().

If all the acceptance set of an SCC but the first one were useless, the
scc_filter() algorithm could abort with a BDD error because of a bug in
the logic.

* src/tgbaalgos/sccfilter.cc (scc_filter): Fix.
* src/tgbatest/sccsimpl.test: Add a test case supplied by Étienne
Renault.

src/tgbatest/sccsimpl.test

@@ -170,3 +170,18 @@ test `grep '^acc' out8.txt | wc -w` = 4
 run 0 ../ltl2tgba -R3 -s -RDS -ks \
   '(G(!((b) R (a)))) R (((c) R (!(d))) U (G((a) | (!(G(e))))))' > out9.txt
 grep 'states: 6$' out9.txt
+
+
+# From Spot 1.1 to 1.1.2, this failed with a BDD error because of
+# a bug in scc_filter().
+run 0 ../ltl2tgba -R3 -k '(a) <-> F(Ga <-> F!(b -> a))' >stdout
+cat >expected <<EOF
+transitions: 21
+states: 8
+total SCCs: 6
+accepting SCCs: 3
+dead SCCs: 0
+accepting paths: 4
+dead paths: 0
+EOF
+diff stdout expected