tests: Fix GSList test to not dereference off the end of an array
tests: Fix GSList test to not dereference off the end of an array

This looks like a typo or copypasta error. As spotted by cppcheck, which
is the code analysis tool GLib deserves, but not the one it needs right
now.

johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib