submodule: catch when submodule is not staged on update

When calling `git_submodule_update` on a submodule, we have to retrieve
the ID of the submodule entry in the index. If the function is called on
a submodule which is only partly initialized, the submodule entry may
not be added to the index yet. This leads to an assert when trying to
look up the blob later on.

Fix the issue by checking if the index actually holds the submodule's
ID and erroring out if it does not.

src/submodule.c

@@ -1212,6 +1212,8 @@ int git_submodule_update(git_submodule *sm, int init, git_submodule_update_optio
 			(error = git_checkout_head(sub_repo, &update_options.checkout_opts)) != 0)
 			goto done;
 	} else {
+		const git_oid *oid;
+
 		/**
 		 * Work dir is initialized - look up the commit in the parent repository's index,
 		 * update the workdir contents of the subrepository, and set the subrepository's
@@ -1220,8 +1222,14 @@ int git_submodule_update(git_submodule *sm, int init, git_submodule_update_optio
 		if ((error = git_submodule_open(&sub_repo, sm)) < 0)
 			goto done;
 
+		if ((oid = git_submodule_index_id(sm)) == NULL) {
+			giterr_set(GITERR_SUBMODULE, "could not get ID of submodule in index");
+			error = -1;
+			goto done;
+		}
+
 		/* Look up the target commit in the submodule. */
-		if ((error = git_object_lookup(&target_commit, sub_repo, git_submodule_index_id(sm), GIT_OBJ_COMMIT)) < 0) {
+		if ((error = git_object_lookup(&target_commit, sub_repo, oid, GIT_OBJ_COMMIT)) < 0) {
 			/* If it isn't found then fetch and try again. */
 			if (error != GIT_ENOTFOUND || !update_options.allow_fetch ||
 				(error = lookup_default_remote(&remote, sub_repo)) < 0 ||