merge: fix NULL pointer dereference when merging nothing into void

When we are on an unborn branch and merging only one foreign parent,
we allow "git merge" to fast-forward to that foreign parent commit.

This codepath incorrectly attempted to dereference the list of
parents that the merge is going to record even when the list is
empty.  It must refuse to operate instead when there is no parent.

All other codepaths make sure the list is not empty before they
dereference it, and are safe.

Reported-by: Jose Ivan B. Vilarouca Filho
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1257,26 +1257,26 @@ 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)
 			die(_("Non-fast-forward commit does not make sense into "
 			    "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

@@ -725,4 +725,14 @@ 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