bc: store only index in p->exits, it's the only thing used there

bc: store only index in p->exits, it's the only thing used there

function                                             old     new   delta
zbc_parse_stmt_possibly_auto                        1967    1964      -3
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-3)               Total: -3 bytes
   text	   data	    bss	    dec	    hex	filename
 981937	    485	   7296	 989718	  f1a16	busybox_old
 981934	    485	   7296	 989715	  f1a13	busybox_unstripped

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