fix crash with “layout default”: ipc.c won’t dump L_DEFAULT layout (#4409)

fixes #4408

testcases/t/312-regress-layout-default.t

@@ -0,0 +1,25 @@
+#!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)
+#
+# Verifies i3 does not crash when using layout default.
+# Ticket: #4408
+# Bug still in: 4.19.2-83-gc8158875
+use i3test;
+
+cmd 'layout default';
+fresh_workspace;
+
+done_testing;