exit_uv: abort if state == 0

It used to crash on NULL pointer dereference, anyways, so make this
just explicit. Also use ERROR to comply with the abort.

Author: MartinPulec

src/host.cpp

@@ -694,7 +694,8 @@ void destroy_root_module(struct module *root_mod) {
  */
 void exit_uv(int status) {
         if (!state_root_static) {
-                log_msg(LOG_LEVEL_WARNING, "%s called without state registered.\n", __func__);
+                MSG(ERROR, "%s called without state registered.\n", __func__);
+                abort();
         }
         state_root_static->exit_status = status;
         state_root_static->broadcast_should_exit();