describe: defensive programming

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

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

builtin/describe.c

@@ -324,6 +324,8 @@ static void describe_commit(struct object_id *oid, struct strbuf *dst)
 	unsigned int unannotated_cnt = 0;
 
 	cmit = lookup_commit_reference(the_repository, oid);
+	if (!cmit)
+		die(_("could not look up commit '%s'"), oid_to_hex(oid));
 
 	n = find_commit_name(&cmit->object.oid);
 	if (n && (tags || all || n->prio == 2)) {