fix a bug in aiger printer

* spot/twaalgos/aiger.cc: here
* tests/core/ltlsynt.test: add a non-regression test

tests/core/ltlsynt.test

@@ -73,6 +73,34 @@ EOF
 ltlsynt --ins='a' --outs='b' -f 'GFa <-> GFb' --aiger > out
 diff out exp
 
+cat >exp <<EOF
+REALIZABLE
+aag 16 1 2 2 13
+2
+4 29
+6 33
+31
+31
+8 5 7
+10 3 8
+12 2 8
+14 4 7
+16 3 14
+18 2 14
+20 5 6
+22 2 20
+24 3 20
+26 17 25
+28 11 26
+30 19 23
+32 13 30
+i0 a
+o0 b
+o1 c
+EOF
+ltlsynt --ins='a' --outs='b,c' -f 'GFa <-> (GFb & GFc)' --aiger > out
+diff out exp
+
 cat >exp <<EOF
 translating formula done
 split inputs and outputs done