get_parent(): defensive programming

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

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

Author: dscho

object-name.c

@@ -1121,7 +1121,7 @@ static enum get_oid_result get_parent(struct repository *r,
 	if (ret)
 		return ret;
 	commit = lookup_commit_reference(r, &oid);
-	if (repo_parse_commit(r, commit))
+	if (!commit || repo_parse_commit(r, commit))
 		return MISSING_OBJECT;
 	if (!idx) {
 		oidcpy(result, &commit->object.oid);