verify_commit_graph(): defensive programming

CodeQL points out that `lookup_commit()` can return NULL values.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

commit-graph.c

@@ -2773,6 +2773,11 @@ static int verify_one_commit_graph(struct commit_graph *g,
 			g->hash_algo);
 
 		graph_commit = lookup_commit(r, &cur_oid);
+		if (!graph_commit) {
+			graph_report(_("failed to look up commit %s for commit-graph"),
+				     oid_to_hex(&cur_oid));
+			continue;
+		}
 		odb_commit = (struct commit *)create_object(r, &cur_oid, alloc_commit_node(r));
 		if (repo_parse_commit_internal(r, odb_commit, 0, 0)) {
 			graph_report(_("failed to parse commit %s from object database for commit-graph"),