smb: client: add NULL check in automount_fullpath

page is checked for null in __build_path_from_dentry_optional_prefix
when tcon->origin_fullpath is not set. However, the check is missing when
it is set.
Add a check to prevent a potential NULL pointer dereference.

Signed-off-by: Ruben Devos <devosruben6@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>