main: Fix warning when opening preferences
If brasero_medium_monitor_get_drive() fails to return a drive, we would get a warning when opening the preferences. This can happen on a system where the optical drive is removable, such as an external USB drive.
#0 0x00007ffff7edcb32 in g_logv () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#1 0x00007ffff7edcda3 in g_log () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#2 0x0000555555575b3d in device_changed_cb (settings=<optimized out>, key=<optimized out>, user_data=<optimized out>) at ../src/sj-prefs.c:293
#3 0x000055555557641c in show_preferences_dialog () at ../src/sj-prefs.c:446