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/lib/i3test/Util.pm

@@ -0,0 +1,47 @@
+package i3test::Util;
+# vim:ts=4:sw=4:expandtab
+
+use strict;
+use warnings;
+use v5.10;
+
+use Exporter qw(import);
+our @EXPORT = qw(
+    slurp
+);
+
+=encoding utf-8
+
+=head1 NAME
+
+i3test::Util - General utility functions
+
+=cut
+
+=head1 EXPORT
+
+=cut
+
+=head2 slurp($fn)
+
+Reads the entire file specified in the arguments and returns the content.
+
+=cut
+sub slurp {
+    my ($file) = @_;
+    my $content = do {
+        local $/ = undef;
+        open my $fh, "<", $file or die "could not open $file: $!";
+        <$fh>;
+    };
+
+    return $content;
+}
+
+=head1 AUTHOR
+
+Michael Stapelberg <michael@i3wm.org>
+
+=cut
+
+1