hush: fix bug where in "var=val func" var's value is not visible in func

hush: fix bug where in "var=val func" var's value is not visible in func

function                                             old     new   delta
unset_local_var                                        -     168    +168
set_vars_all_and_save_old                              -      87     +87
get_ptr_to_local_var                                   -      77     +77
free_strings_and_unset                                 -      53     +53
builtin_export                                       266     274      +8
get_local_var_value                                   31      33      +2
putenv_all                                            27       -     -27
free_strings_and_unsetenv                             53       -     -53
get_local_var                                         68       -     -68
run_list                                            2475    2350    -125
builtin_unset                                        380     220    -160
------------------------------------------------------------------------------
(add/remove: 4/3 grow/shrink: 2/2 up/down: 395/-433)          Total: -38 bytes

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