sdap_extend_map: make sure memory can be freed
    
    If there is an error after calling talloc_realloc() the caller cannot
    free the memory properly because neither src_map nor _map were pointing
    to a valid memory location. With this patch _map will always point to
    the current valid location so that it can always be used with
    talloc_free().
    
    Reviewed-by: Petr Cech <pcech@redhat.com>
    (cherry picked from commit 08bf6b4a281ef4308119dccbba4e86cf28b505d2)