libbb/login/su: do not sanitize shell name twice

function                                             old     new   delta
setup_environment                                    191     205     +14
login_main                                          1002     987     -15
su_main                                              474     458     -16
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(add/remove: 0/0 grow/shrink: 1/2 up/down: 14/-31)            Total: -17 bytes

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