vk-driver: Don't crash when gdk keysm conversion fails

gdk_wayland_keymap_get_entries_for_keyval is supposed to return FALSE when conversion fails but currently doesn't. See

GNOME/gtk!6492 (merged)

so check n_keys as well.

We can also support the KEY_{Down,Up,Left,Right} symbols with a little extra effort via

gsettings set sm.puri.phosh.osk.Terminal shortcuts "['Down', 'Up', 'Left', 'Right']"

Signed-off-by: Guido Günther agx@sigxcpu.org Closes: #24 (closed)