otf_product: fix deletion of iter_cache_

Fixes #152, reported by Valentin Iovene.

* spot/twa/twaproduct.cc (~twa_product): Delete iter_cache_.
* tests/python/otfcrash.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention the bug.

tests/python/otfcrash.py

@@ -0,0 +1,39 @@
+import spot
+import spot.ltsmin
+import tempfile
+import shutil
+import sys
+
+# this test requires divine with --ltsmin support
+if shutil.which("divine") == None:
+    sys.exit(77)
+import subprocess
+out = subprocess.check_output(['divine', 'compile',
+                               '--help'], stderr=subprocess.STDOUT)
+if b'LTSmin' not in out:
+    sys.exit(77)
+
+# the test case actually starts here
+with tempfile.NamedTemporaryFile(dir='.', suffix='.dve') as fp:
+    fp.write(b"""int f = 3;
+process R {
+  int p = 1, found = 0;
+  state i, e;
+  init i;
+  trans
+    i -> i {guard p != f; effect p = p + 1;},
+    i -> e {guard p == f; effect found = 1;},
+    e -> e {};
+}
+system async;
+""")
+    fp.flush()
+    m = spot.ltsmin.load(fp.name)
+
+    def modelcheck(formula, model):
+        a = spot.translate(formula)
+        k = m.kripke([ap.ap_name() for ap in a.ap()])
+        p = spot.otf_product(k, a)
+        return p.is_empty()
+
+    assert(modelcheck('X "R.found"', m) == True)