Fix dead assignments

"Value stored to 'stmt' is never read"
"Value stored to 'grp_to_use' is never read"

And change 'grp' to 'group' if we're here.

Signed-off-by: Ran Benita <ran234@gmail.com>
ast: add error handling to XkbFileFromComponents

And try to not repeat ourselves.

Signed-off-by: Ran Benita <ran234@gmail.com>