nfc: netlink: Add check on NFC_ATTR_VENDOR_DATA

NFC_ATTR_VENDOR_DATA is an optional vendor_cmd argument.
The current code was potentially using a non existing argument
leading to potential catastrophic results.

Cc: stable@vger.kernel.org
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>