fixed CRL check return value on revoked certificates

fixed possible refcounting bugs
generic return_null() implementation

src/libstrongswan/utils.c

@@ -63,6 +63,14 @@ void memxor(u_int8_t dest[], u_int8_t src[], size_t n)
 	}
 }
 
+/**
+ * return null
+ */
+void *return_null()
+{
+	return NULL;
+}
+
 /**
  * We use a single mutex for all refcount variables. This
  * is not optimal for performance, but the critical section