ctrl: Set a generic reply when it hasn'n been set

When verification failed and the reply string was not updated, the
message "Someone forgot to fill in the reply." was shown instead
of the default "Value failed verification." message.

This patch changes the default reply handling in ctrl_cmd_handle()
by setting the reply to NULL initially and then checking it at the
end. If it hasn't been set, a generic message is assigned and an
error is logged.
diff --git a/openbsc/tests/ctrl_test_runner.py b/openbsc/tests/ctrl_test_runner.py
index 24732e0..dfdec40 100644
--- a/openbsc/tests/ctrl_test_runner.py
+++ b/openbsc/tests/ctrl_test_runner.py
@@ -159,6 +159,10 @@
         self.assertEquals(r['mtype'], 'ERROR')
         self.assertEquals(r['error'], 'Command not found')
 
+        r = self.do_set('rf_locked', '999')
+        self.assertEquals(r['mtype'], 'ERROR')
+        self.assertEquals(r['error'], 'Value failed verification.')
+
         r = self.do_get('bts')
         self.assertEquals(r['mtype'], 'ERROR')
         self.assertEquals(r['error'], 'Error while parsing the index.')