Bugfix: resize window check should check for null

When checking the window type for a resize command, first check to see
if the window property is null before checking whether or not it is a
dock window. The window may be null in the case it is a branch
container.

fixes #1220

testcases/t/224-regress-resize-branch.t

@@ -0,0 +1,33 @@
+#!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)
+#
+# Test that i3 does not crash when resizing a split container
+# Ticket: #1220
+# Bug still in: 4.7.2-128-g702906d
+use i3test;
+
+open_window;
+open_window;
+
+cmd 'split h';
+
+open_window;
+
+cmd 'focus parent, resize grow left';
+
+does_i3_live;
+
+done_testing;