filemanager: Add assertion checks to prevent dereferencing ptrs

* clang static analysis shows gaps where these calls could
  rereferencce pointers. Perform an assertion check.

plugins/filebrowser/pluma-file-browser-store.c

@@ -2480,6 +2480,8 @@ set_virtual_root_from_node (PlumaFileBrowserStore * model,
 	GSList *copy;
 	GtkTreePath *empty = NULL;
 
+	g_assert (node != NULL);
+
 	prev = node;
 	next = prev->parent;
 

plugins/filebrowser/pluma-file-browser-widget.c

@@ -1640,6 +1640,8 @@ jump_to_location (PlumaFileBrowserWidget * obj, GList * item,
 
 	obj->priv->changing_location = TRUE;
 
+	g_assert (obj->priv->current_location != NULL);
+
 	loc = (Location *) (obj->priv->current_location->data);
 
 	/* Set the new root + virtual root */