Fix segmentation fault

As reported by Maxim Zhukov, the commit 9ca7710 ("Fix memory leak in
main view (#931)", 01-06-2019) causes a SIGSEGV when holding the key
'Stage/Unstage current line' in stage view.

Add a check to avoid null pointer dereference.

Fixes #971

src/graph-v2.c

@@ -257,7 +257,8 @@ done_graph(struct graph *graph_ref)
 {
 	struct graph_v2 *graph = graph_ref->private;
 
-	htab_delete(graph->colors.id_map);
+	if (graph->colors.id_map)
+		htab_delete(graph->colors.id_map);
 
 	free(graph);