orinoco_usb: potential null dereference

Smatch complains that "upriv->read_urb" gets dereferenced before
checking for NULL.  It turns out that it's possible for
"upriv->read_urb" to be NULL so I added checks around the dereferences.

Also I remove an "if (upriv->bap_buf != NULL)" check because
"kfree(NULL) is OK.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>