vfs_default: Protect vfs_getxattrat_done() from accessing a freed req pointer

vfs_default: Protect vfs_getxattrat_done() from accessing a freed req pointer

If the fsp is forced closed by a SHUTDOWN_CLOSE whilst the request is in
flight (share forced closed by smbcontrol), then we set state->req = NULL in the
state destructor.

The existing state destructor prevents the state memory from being freed, so
when the thread completes and calls vfs_getxattrat_done(), just throw away the result
if state->req == NULL.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14301

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>