bc: remove BC_STATUS_EOF (again), the condition is detectable as len==0

bc: remove BC_STATUS_EOF (again), the condition is detectable as len==0

function                                             old     new   delta
bc_read_line                                         147     129     -18
bc_vm_run                                            618     591     -27
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-45)             Total: -45 bytes
   text	   data	    bss	    dec	    hex	filename
 980802	    485	   7296	 988583	  f15a7	busybox_old
 980757	    485	   7296	 988538	  f157a	busybox_unstripped

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