acc: strengthen is_generalized_rabin() and is_generalized_streett()

* spot/twa/acc.cc: These functions were segfaulting on acceptance
conditions such as "Acceptance: 3 t" or "Acceptance: 3 f".
The issue was revealed on branch "next" by the change that print_dot()
display the acceptance condition by default, but we want the fix on
master as well.
* NEWS: Mention the issue.

NEWS

@@ -11,6 +11,12 @@ New in spot 2.5.1.dev (not yet released)
     update an external structure that references states of the twa
     that we want to purge.
 
+  Bugs fixed:
+
+  - acc_cond::is_generalized_rabin() and
+    acc_cond::is_generalized_streett() would segfault on weird
+    acceptance conditions such as "3 t" or "3 f".
+
 New in spot 2.5.1 (2018-02-20)
 
   Library:

spot/twa/acc.cc

@@ -660,7 +660,10 @@ namespace spot
         pairs.resize(num_);
         return true;
       }
-    if (code_.is_t()
+    // "Acceptance: 0 f" is caught by is_generalized_rabin() above.
+    // Therefore is_f() below catches "Acceptance: n f" with n>0.
+    if (code_.is_f()
+        || code_.is_t()
         || code_.back().sub.op != acc_op::Or)
       return false;
 
@@ -741,7 +744,10 @@ namespace spot
         pairs.resize(num_);
         return true;
       }
-    if (code_.is_f()
+    // "Acceptance: 0 t" is caught by is_generalized_buchi() above.
+    // Therefore is_t() below catches "Acceptance: n t" with n>0.
+    if (code_.is_t()
+        || code_.is_f()
         || code_.back().sub.op != acc_op::And)
       return false;