hush: fix a bug with backslashes improperly handled in unquoted variables.

hush: fix a bug with backslashes improperly handled in unquoted variables.
 with previous patch:

function                                             old     new   delta
parse_stream                                        1638    1758    +120
expand_on_ifs                                         97     174     +77
free_pipe                                            206     237     +31
setup_redirect                                       217     220      +3
setup_redirects                                      143     144      +1
done_word                                            698     688     -10
free_strings                                          38       -     -38
expand_variables                                    1451    1403     -48
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(add/remove: 0/1 grow/shrink: 5/2 up/down: 232/-96)           Total: 136 bytes