mdoc_valid_post() may indirectly call roff_node_unlink() which may

mdoc_valid_post() may indirectly call roff_node_unlink() which may
set ROFF_NEXT_CHILD, which is desirable for the final call to
mdoc_valid_post() - in case the target itself gets deleted, the
parse point may need this adjustment - but not for the intermediate
calls - if intermediate nodes get deleted, that mustn't clobber the
parse point.  So move setting ROFF_NEXT_SIBLING to the proper place
in rew_last().

This fixes the assertion failure in jsg@'s afl test case 108/Apr27.