Made nvr_close() no longer attempt to free a NULL NVR pointer.

src/nvr_at.c

@@ -1010,6 +1010,7 @@ nvr_at_close(void *priv)
     timer_disable(&local->update_timer);
     timer_disable(&nvr->onesec_time);
 
+    if (nvr != NULL) {
 	if (nvr->fn != NULL)
 		free(nvr->fn);
 
@@ -1017,6 +1018,7 @@ nvr_at_close(void *priv)
 		free(nvr->data);
 
 	free(nvr);
+    }
 
     if (nvr_at_inited == 1)
 	nvr_at_inited = 0;