bc: shrink bc_program_index()

bc: shrink bc_program_index()

function                                             old     new   delta
bc_program_index                                      66      47     -19
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-19)             Total: -19 bytes
   text	   data	    bss	    dec	    hex	filename
 981418	    485	   7296	 989199	  f180f	busybox_old
 981399	    485	   7296	 989180	  f17fc	busybox_unstripped

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