Fix segfault when calling "i3 -C".

Commit 287a0b4 introduced a segfault when validating the i3 config
as the root_screen will not be set in this case, causing a null
pointer dereference.

fixes #2144

testcases/t/262-config-validation.t

@@ -0,0 +1,39 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • http://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • http://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • http://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)
+#
+# Ensures that calling i3 with the -C switch works (smoke test).
+# Ticket: #2144
+use i3test i3_autostart => 0;
+use POSIX ":sys_wait_h";
+use Time::HiRes qw(sleep);
+
+my $config = <<EOT;
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+
+invalid
+EOT
+
+my $exit_code = launch_with_config($config, validate_config => 1);
+isnt($exit_code, 0, 'i3 exited with an error code');
+
+my $log = get_i3_log;
+
+# We don't care so much about the output (there are tests for this), but rather
+# that we got correct output at all instead of, e.g., a segfault.
+ok($log =~ /Expected one of these tokens/, 'an invalid config token was found');
+
+done_testing;