Fix crash with moving container that is to be flattened

Same can happen with move_to_output_directed but it is not so easy to
write a test about it.

Fixes #3831

testcases/t/309-crash-move-parent.t

@@ -0,0 +1,43 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • https://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • https://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • https://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
+# Test that moving a container that is to be flattened does not crash i3
+# Ticket: #3831
+# Bug still in: 4.17-199-ga638e0408
+use i3test;
+
+cmp_tree(
+    msg => 'Moving a redundant container that is to be flattened does not crash i3',
+    layout_before => 'a H[V[b* c]]',    # Not very attached to this result but
+    layout_after => 'H[a] b* c',        # mainly checking if the crash happens.
+    cb => sub {
+        cmd 'focus parent, focus parent, move down';
+        does_i3_live;
+        is(get_ws(focused_ws)->{layout}, 'splitv', 'Workspace changed to splitv');
+    });
+
+cmp_tree(
+    msg => "Same but create the redundant container with a 'split h' command",
+    layout_before => 'a V[b* c]',
+    layout_after => 'H[a] b* c',
+    cb => sub {
+        cmd 'focus parent, split h, focus parent, move down';
+        does_i3_live;
+        is(get_ws(focused_ws)->{layout}, 'splitv', 'Workspace changed to splitv');
+    });
+
+
+done_testing;