Merge branch 'jv/merge-nothing-into-void' into maint

"git merge FETCH_HEAD" dereferenced NULL pointer when merging
nothing into an unborn history (which is arguably unusual usage,
which perhaps was the reason why nobody noticed it).

* jv/merge-nothing-into-void:
  merge: fix NULL pointer dereference when merging nothing into void

builtin/merge.c

@@ -1257,12 +1257,12 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			builtin_merge_options);
 
 	if (!head_commit) {
-		struct commit *remote_head;
 		/*
 		 * If the merged head is a valid one there is no reason
 		 * to forbid "git merge" into a branch yet to be born.
 		 * We do the same for "git pull".
 		 */
+		unsigned char *remote_head_sha1;
 		if (squash)
 			die(_("Squash commit into empty head not supported yet"));
 		if (fast_forward == FF_NO)
@@ -1270,13 +1270,13 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			    "an empty head"));
 		remoteheads = collect_parents(head_commit, &head_subsumed,
 					      argc, argv, NULL);
-		remote_head = remoteheads->item;
-		if (!remote_head)
+		if (!remoteheads)
 			die(_("%s - not something we can merge"), argv[0]);
 		if (remoteheads->next)
 			die(_("Can merge only exactly one commit into empty head"));
-		read_empty(remote_head->object.oid.hash, 0);
-		update_ref("initial pull", "HEAD", remote_head->object.oid.hash,
+		remote_head_sha1 = remoteheads->item->object.oid.hash;
+		read_empty(remote_head_sha1, 0);
+		update_ref("initial pull", "HEAD", remote_head_sha1,
 			   NULL, 0, UPDATE_REFS_DIE_ON_ERR);
 		goto done;
 	}

t/t7600-merge.sh

@@ -753,4 +753,14 @@ test_expect_success 'merge detects mod-256 conflicts (resolve)' '
 	test_must_fail git merge -s resolve master
 '
 
+test_expect_success 'merge nothing into void' '
+	git init void &&
+	(
+		cd void &&
+		git remote add up .. &&
+		git fetch up &&
+		test_must_fail git merge FETCH_HEAD
+	)
+'
+
 test_done