hush: explain parsing context structure

hush: explain parsing context structure
 plug leak in setup_redirect on error path

function                                             old     new   delta
done_command                                          84      86      +2
done_word                                            657     658      +1
done_pipe                                            105     106      +1
initialize_context                                    39      38      -1
setup_redirect                                       219     212      -7
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(add/remove: 0/0 grow/shrink: 3/2 up/down: 4/-8)               Total: -4 bytes