bc: shrink s() in library again

bc: shrink s() in library again

function                                             old     new   delta
bc_lib                                              1596    1586     -10
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-10)             Total: -10 bytes
   text	   data	    bss	    dec	    hex	filename
 980152	    485	   7296	 987933	  f131d	busybox_old
 980142	    485	   7296	 987923	  f1313	busybox_unstripped

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