bc: simplify filename saving in zbc_vm_file()

bc: simplify filename saving in zbc_vm_file()

function                                             old     new   delta
bc_vm_run                                            433     423     -10
zbc_vm_file                                          187     174     -13
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-23)             Total: -23 bytes
   text	   data	    bss	    dec	    hex	filename
 980350	    485	   7296	 988131	  f13e3	busybox_old
 980327	    485	   7296	 988108	  f13cc	busybox_unstripped

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