keyboard: Avoid segfault due to no "data" for `custom_shortcut_add_row`

Ultimately it would be good to get rid of the use of
`g_object_set_data`/`g_object_get_data` here, but this works for now.

Fixes GNOME/gnome-control-center#1373.
10 jobs for keyboard-segfault-41 in 60 minutes and 31 seconds (queued for 45 seconds)
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