We can return XKB_MOD_INVALID for AltGr which rightly gets flagged by
sanitisers as an overly wide shift attempt. Properly check the return
type and leave the bitmap as zero in that case. Tested output before
and after is unchanged with the gb and ara keymaps.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-7-alex.bennee@linaro.org>