commit-graph: fix memory leak

Free the commit graph when verify_commit_graph_lite() reports an error.
Credit to OSS-Fuzz for finding this leak.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -267,8 +267,10 @@ struct commit_graph *parse_commit_graph(void *graph_map, int fd,
 		last_chunk_offset = chunk_offset;
 	}
 
-	if (verify_commit_graph_lite(graph))
+	if (verify_commit_graph_lite(graph)) {
+		free(graph);
 		return NULL;
+	}
 
 	return graph;
 }