bc: simplify string-tonumber conversion code

bc: simplify string-tonumber conversion code

function                                             old     new   delta
bc_program_num                                      1108     983    -125
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-125)           Total: -125 bytes
   text	   data	    bss	    dec	    hex	filename
 985831	    477	   7296	 993604	  f2944	busybox_old
 985706	    477	   7296	 993479	  f28c7	busybox_unstripped

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