ltlsynt: fix syntax error handling

Fixes #477.

* bin/common_finput.cc (job_processor::process_string): Fix return
value.
* bin/ltlsynt.cc: Fix handling of syntax errors.  While we are here,
make sure game_info is destroyed before Spot's globals.
* tests/core/ltlsynt.test: Add a test case.

tests/core/ltlsynt.test

@@ -509,4 +509,9 @@ o1 o1
 EOF
 ltlsynt -f "G((i0 && i1)<->X(o0)) && G((i2|i3)<->X(o1))" --outs="o0,o1"\
   --aiger=isop --algo=lar -x"specification-decomposition=1" >out
-diff out exp
+diff out exp
+
+# Issue #477
+ltlsynt -f 'a U (b' 2>err && exit 1
+test $? -eq 2
+test `wc -l <err` -eq 4