printf: short-circuit output when argument to %b includes \c

printf: short-circuit output when argument to %b includes \c

printf wasn't correctly handling \c in an argument to the %b format
specifier.

   printf %bXX OK\\c

returned 'OK\cXX' rather than the expected 'OK'.

function                                             old     new   delta
printf_main                                          886     899     +13

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>