Bugfix: Fix floating resize for floating split containers (+test)

Fixes: #588

testcases/t/184-regress-float-split-resize.t

@@ -0,0 +1,23 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Regression: resizing a floating split container leads to a crash.
+# (Ticket #588, present until 4412ccbe5a4fad8a4cd594e6f10f937515a4d37c)
+#
+use i3test;
+
+my $tmp = fresh_workspace;
+
+my $first = open_window;
+cmd 'split v';
+my $second = open_window;
+
+cmd 'focus parent';
+cmd 'floating toggle';
+cmd 'layout stacking';
+
+cmd 'resize grow up 10 px or 10 ppt';
+
+does_i3_live;
+
+done_testing;