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

drivers/hid/hidraw.c

@@ -109,6 +109,12 @@ static ssize_t hidraw_write(struct file *file, const char __user *buffer, size_t
 	int ret = 0;
 
 	mutex_lock(&minors_lock);
+
+	if (!hidraw_table[minor]) {
+		ret = -ENODEV;
+		goto out;
+	}
+
 	dev = hidraw_table[minor]->hid;
 
 	if (!dev->hid_output_raw_report) {