hush: fix "export PS1=xyz" and "local PS1=xyz" messing up prompt

hush: fix "export PS1=xyz" and "local PS1=xyz" messing up prompt

function                                             old     new   delta
helper_export_local                                  215     253     +38
leave_var_nest_level                                 107     127     +20
run_pipe                                            1840    1857     +17
handle_changed_special_names                         101     105      +4
shell_builtin_read                                  1399    1398      -1
done_word                                            767     766      -1
parse_stream                                        2249    2245      -4
set_local_var                                        437     430      -7
is_well_formed_var_name                               66       -     -66
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 4/4 up/down: 79/-79)              Total: 0 bytes
   text	   data	    bss	    dec	    hex	filename
 952376	    485	   7296	 960157	  ea69d	busybox_old
 952400	    485	   7296	 960181	  ea6b5	busybox_unstripped

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