ieee802154: Prevent user from crashing the host

Avoid crashing the machine by checking
info->attrs[NL802154_ATTR_SCAN_TYPE] presence before de-referencing it,
which was the primary intend of the blamed patch.

Reported-by: Sanan Hasanov <sanan.hasanov@Knights.ucf.edu>
Suggested-by: Eric Dumazet <edumazet@google.com>
Fixes: a0b61066

 ("ieee802154: Convert scan error messages to extack")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20230301154450.547716-1-miquel.raynal@bootlin.com


Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>