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>

fs/overlayfs/export.c

@@ -108,6 +108,9 @@ static int ovl_d_to_fh(struct dentry *dentry, char *buf, int buflen)
 
 	/* Encode an upper or origin file handle */
 	fh = ovl_encode_fh(origin ?: ovl_dentry_upper(dentry), !origin);
+	err = PTR_ERR(fh);
+	if (IS_ERR(fh))
+		goto fail;
 
 	err = -EOVERFLOW;
 	if (fh->len > buflen)