polycubed: fix segfault on init (#114)

* polycubed: fix segfault on init

When SIGINT was received at initialization time there was segfault because
the handler tried to delete the core and rest server that were not allocated
yet.  This commit adds a check to be sure that core and rest server are
allocated when the shutdown handler is invoked.

Fixes: 0df5d81 ("Initial commit")

Signed-off-by: Mauricio Vasquez B <mauriciovasquezbernal@gmail.com>

* Update polycubed.cpp

src/polycubed/src/polycubed.cpp

@@ -60,10 +60,13 @@ void shutdown() {
     return;
   // Service controllers depend on Rest Server. It is required to remove them
   // before clearing the rest server in order to avoid segmentation faults.
-  core->clear_servicectrl_list();
-  restserver->shutdown();
-  logger->debug("rest was shutdown");
-  delete core;
+  if (core && restserver) {
+    core->clear_servicectrl_list();
+    restserver->shutdown();
+    logger->debug("rest server shutdown");
+    delete core;
+    delete restserver;
+  }
   logger->info("polycubed is shutting down. Bye!");
   done = true;
 }