Bad code path in --random: can't specify directory on command line

[daf]

Noticed in warnings from #84:

main.cc: In function ‘int set_bg_once(Config*, SetBG*, Glib::ustring, int, SetBG::SetMode, bool, Gdk::Color, bool)’:
main.cc:49:40: warning: left operand of comma operator is a reference, not call, to function ‘Glib::file_test’ [-Waddress]
             if (Glib::file_test, Glib::FILE_TEST_IS_DIR) {
                                        ^
main.cc:49:40: warning: left operand of comma operator has no effect [-Wunused-value]

This is certainly not working properly. Code is main.cc:49