lineedit: fix another corner case with bad unicode input

lineedit: fix another corner case with bad unicode input

function                                             old     new   delta
read_key                                             607     646     +39
readit                                                50      55      +5
getch_nowait                                         290     295      +5
hash_find                                            233     234      +1
xstrtoul_range_sfx                                   231     230      -1
passwd_main                                         1058    1056      -2
builtin_exit                                          45      43      -2
cmp_main                                             649     645      -4
lineedit_read_key                                    257     245     -12
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(add/remove: 0/0 grow/shrink: 4/5 up/down: 50/-21)             Total: 29 bytes

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