ath9k_htc: fix NULL-deref at probe

commit ebeb36670ecac36c179b5fb5d5c88ff03ba191ec upstream.

Make sure to check the number of endpoints to avoid dereferencing a
NULL-pointer or accessing memory beyond the endpoint array should a
malicious device lack the expected endpoints.

Fixes: 36bcce43 ("ath9k_htc: Handle storage devices")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>