Bluetooth: fix return value check

In case of error, the function bt_skb_alloc() returns NULL pointer
not ERR_PTR(). The IS_ERR() test in the return value check should
be replaced with NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>