bc: no need to "set to zero" BcNum after bc_num_init(), it already is

bc: no need to "set to zero" BcNum after bc_num_init(), it already is

function                                             old     new   delta
bc_num_init                                           48      49      +1
bc_program_num                                      1115    1108      -7
bc_program_exec                                     4098    4081     -17
bc_vm_init                                           786     768     -18
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/3 up/down: 1/-42)             Total: -41 bytes
   text	   data	    bss	    dec	    hex	filename
 985872	    477	   7296	 993645	  f296d	busybox_old
 985831	    477	   7296	 993604	  f2944	busybox_unstripped

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