HID: cougar: fix slab-out-of-bounds Read in cougar_report_fixup

report_fixup for the Cougar 500k Gaming Keyboard was not verifying
that the report descriptor size was correct before accessing it

Reported-by:  <syzbot+24c0361074799d02c452@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=24c0361074799d02c452


Signed-off-by: Camila Alvarez <cam.alvarez.i@gmail.com>
Reviewed-by: Silvan Jegen <s.jegen@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>