Bugfix: Fix crash on '[class="i3bar"] focus' (+test) (Thanks f8l)

Fixes #575

testcases/t/182-regress-focus-dock.t

@@ -0,0 +1,21 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Regression test: Focusing a dock window should just do nothing, not crash i3.
+# See ticket http://bugs.i3wm.org/575
+# Wrong behaviour manifested itself up to (including) commit
+# 340592a532b5259c3a3f575de5a9639fad4d1459
+#
+use i3test;
+
+fresh_workspace;
+
+my $window = open_window(
+    window_type => $x->atom(name => '_NET_WM_WINDOW_TYPE_DOCK'),
+);
+
+cmd '[title="' . $window->name . '"] focus';
+
+does_i3_live;
+
+done_testing;