HID: hidraw, fix a NULL pointer dereference in hidraw_write

BUG: unable to handle kernel NULL pointer dereference at 0000000000000028
IP: [<ffffffffa0f0a625>] hidraw_write+0x3b/0x116 [hid]
[...]

This is reproducible by disconnecting the device while userspace writes
to dev node in a loop and doesn't check return values in order to exit
the loop.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Cc: stable@kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>