FreeBSD: Prevent NULL pointer dereference of resid

spa_config_load() passes NULL into resid when doing zfs_file_read().
This would trip over when vfs.zfs.autoimport_disable=0.

Sponsored by: The FreeBSD Foundation
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Allan Jude <[email protected]>
Signed-off-by: Ka Ho Ng <[email protected]>
Closes openzfs#11149

Author: khng300

module/os/freebsd/zfs/zfs_file_os.c

@@ -158,7 +158,8 @@ zfs_file_read_impl(zfs_file_t *fp, void *buf, size_t count, loff_t *offp,
 	rc = fo_read(fp, &auio, td->td_ucred, FOF_OFFSET, td);
 	if (rc)
 		return (SET_ERROR(rc));
-	*resid = auio.uio_resid;
+	if (resid)
+		*resid = auio.uio_resid;
 	*offp += count - auio.uio_resid;
 	return (SET_ERROR(0));
 }