Bluetooth: virtio_bt: add missing null pointer check on alloc_skb call return

The call to alloc_skb with the GFP_KERNEL flag can return a null sk_buff
pointer, so add a null check to avoid any null pointer deference issues.

Addresses-Coverity: ("Dereference null return value")
Fixes: afd2daa2 ("Bluetooth: Add support for virtio transport driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>