hush: make pun_pipe loop clearer; fix "cmd | var=`cmd` | cmd" handling

hush: make pun_pipe loop clearer; fix "cmd | var=`cmd` | cmd" handling

function                                             old     new   delta
free_strings                                           -      38     +38
pseudo_exec_argv                                     161     171     +10
free_pipe                                            227     205     -22
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(add/remove: 1/0 grow/shrink: 1/1 up/down: 48/-22)             Total: 26 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>