nfc: Ensure presence of NFC_ATTR_FIRMWARE_NAME attribute in nfc_genl_fw_download()

Check that the NFC_ATTR_FIRMWARE_NAME attributes are provided by
the netlink client prior to accessing them.This prevents potential
unhandled NULL pointer dereference exceptions which can be triggered
by malicious user-mode programs, if they omit one or both of these
attributes.

Similar to commit a0323b97 ("nfc: Ensure presence of required attributes in the activate_target handler").

Fixes: 9674da87 ("NFC: Add firmware upload netlink command")
Signed-off-by: Defang Bo <bodefang@126.com>
Link: https://lore.kernel.org/r/1603107538-4744-1-git-send-email-bodefang@126.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>