libi3: validate UTF8 strings

Will validate container / window titles.

Fixes #3156.

testcases/t/300-restart-non-utf8.t

@@ -0,0 +1,34 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • https://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • https://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • https://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)
+#
+# Verify that i3 does not crash when restart is issued while a window with a
+# title that contains non-UTF8 characters is open.
+# Ticket: #3156
+# Bug still in: 4.15-241-g9dc4df81
+use i3test;
+
+my $ws = fresh_workspace;
+open_window(name => "\x{AA} <-- invalid");
+
+cmd 'restart';
+does_i3_live;
+
+# Confirm that the invalid character got replaced with U+FFFD - "REPLACEMENT
+# CHARACTER"
+cmd '[title="^' . "\x{fffd}" . ' <-- invalid$"] fullscreen enable';
+is_num_fullscreen($ws, 1, 'title based criterion works');
+
+done_testing;