modesetting: fix conn_id termination and potential overrun by 1 byte

Noticed when porting this logic to xf86-video-nouveau, and valgrind complained about conditional jump based on uninitialized data.

Signed-off-by: Ilia Mirkin imirkin@alum.mit.edu Reviewed-by: Pekka Paalanen pekka.paalanen@collabora.com