libbb: shrink obscure()

libbb: shrink obscure()

function                                             old     new   delta
string_checker_helper                                 59      45     -14
string_checker                                       116      98     -18
obscure                                              367     204    -163
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-195)           Total: -195 bytes

Signed-off-by: Bob Dunlop <bob.dunlop@xyzzy.org.uk>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>