usb: gadget: FunctionFS: dereference ffs_dev conditionally

ffs_dev->ffs_release_dev_callback should be accessed only if ffs_dev
is not NULL.
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_fs.c

@@ -2590,11 +2590,12 @@ static void ffs_release_dev(struct ffs_data *ffs_data)
 	ffs_dev_lock();
 
 	ffs_dev = ffs_data->private_data;
-	if (ffs_dev)
+	if (ffs_dev) {
 		ffs_dev->mounted = false;
 
 		if (ffs_dev->ffs_release_dev_callback)
 			ffs_dev->ffs_release_dev_callback(ffs_dev);
+	}
 
 	ffs_dev_unlock();
 }