Bluetooth: clean up error pointer checking

The bt_skb_sendmsg() function can't return NULL so there is no need to
check for that.  Several of these checks were removed previously but
this one was missed.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>