Bluetooth: Fix bug when retransmitting I-frames

If there is no frames to retransmit l2cap was crashing the kernel, now
we check if the queue is empty first.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>