ovl: check ERR_PTR() return value from ovl_encode_fh()

Another fix for an issue reported by 0-day robot.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 8ed5eec9

 ("ovl: encode pure upper file handles")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>