bc: p->exits.func is never zero, do not check for that

bc: p->exits.func is never zero, do not check for that

function                                             old     new   delta
zbc_parse_stmt_possibly_auto                        1978    1967     -11
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-11)             Total: -11 bytes
   text	   data	    bss	    dec	    hex	filename
 981948	    485	   7296	 989729	  f1a21	busybox_old
 981937	    485	   7296	 989718	  f1a16	busybox_unstripped

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