bc: disallow empty statement as function body

bc: disallow empty statement as function body

	$ bc
	define z() <cr>
	<cr>
	bc: no statement after 'define'

function                                             old     new   delta
zbc_parse_stmt_possibly_auto                        2239    2245      +6
zbc_vm_process                                       589     594      +5
zbc_parse_stmt_fail_if_bare_NLINE                     25      28      +3
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 14/0)               Total: 14 bytes
   text	   data	    bss	    dec	    hex	filename
 982216	    485	   7296	 989997	  f1b2d	busybox_old
 982237	    485	   7296	 990018	  f1b42	busybox_unstripped

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