scc_info: detect incorrect initial state

* spot/twaalgos/sccinfo.cc: Here.
* tests/python/sccinfo.py: Test it.

spot/twaalgos/sccinfo.cc

@@ -75,6 +75,11 @@ namespace spot
       options_(options)
   {
     unsigned n = aut->num_states();
+
+    if (initial_state != -1U && n <= initial_state)
+      throw std::runtime_error
+        ("scc_info: supplied initial state does not exist");
+
     sccof_.resize(n, -1U);
 
     if (!!(options & scc_info_options::TRACK_STATES_IF_FIN_USED)

tests/python/sccinfo.py

@@ -22,6 +22,12 @@ import spot
 
 a = spot.translate('(Ga -> Gb) W c')
 
+try:
+    si = spot.scc_info(a, 10)
+    exit(2)
+except RuntimeError as e:
+    assert "initial state does not exist" in str(e)
+
 si = spot.scc_info(a)
 n = si.scc_count()
 assert n == 4