bc: further simplify string-to-number conversion code

bc: further simplify string-to-number conversion code

function                                             old     new   delta
bc_program_index                                      66      64      -2
bc_program_num                                       983     963     -20
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-22)             Total: -22 bytes
   text	   data	    bss	    dec	    hex	filename
 985706	    477	   7296	 993479	  f28c7	busybox_old
 985684	    477	   7296	 993457	  f28b1	busybox_unstripped

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