Fix memory leak in close_display_cb()

Fixes memory leak seen with asan (as well as valgrind)

==392857==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 16 byte(s) in 1 object(s) allocated from: #0 0x7f4e136d92ef in malloc (/lib64/libasan.so.8+0xd92ef) (BuildId: 7fcb7759bc17ef47f9682414b6d99732d6a6ab0c) #1 (closed) 0x7f4e130db619 in g_malloc (/lib64/libglib-2.0.so.0+0x61619) (BuildId: b6cf99186f67796c572114f06c6ee38c300c212b) #2 (closed) 0x7f4e130f7244 in g_slice_alloc (/lib64/libglib-2.0.so.0+0x7d244) (BuildId: b6cf99186f67796c572114f06c6ee38c300c212b) #3 (closed) 0x7f4e130f7478 in g_slist_copy_deep (/lib64/libglib-2.0.so.0+0x7d478) (BuildId: b6cf99186f67796c572114f06c6ee38c300c212b) #4 (closed) 0x7f4e13d0c40e in close_display_cb (/lib64/libpangoxft-1.0.so.0+0x640e) (BuildId: cc253836024fa87064b295dbb324a1edf5a747ad) #5 (closed) 0x7f4e12e7bd03 in XCloseDisplay (/lib64/libX11.so.6+0x28d03) (BuildId: d7bb6ea97159381d17979237bb7c97ea885d9383)