bc: avoid successive bc_vec_item(&func->autos, i)

bc: avoid successive bc_vec_item(&func->autos, i)

function                                             old     new   delta
zbc_func_insert                                       97     100      +3
zbc_program_call                                     353     354      +1
zbc_program_exec                                    4085    4078      -7
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 4/-7)               Total: -3 bytes
   text	   data	    bss	    dec	    hex	filename
 981403	    485	   7296	 989184	  f1800	busybox_old
 981400	    485	   7296	 989181	  f17fd	busybox_unstripped

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