bc: pull zbc_lex_next() call out of zbc_parse_operator() into one caller that uses it

bc: pull zbc_lex_next() call out of zbc_parse_operator() into one caller that uses it

function                                             old     new   delta
bc_parse_operator                                      -     144    +144
bc_parse_expr_empty_ok                              1788    1792      +4
zbc_parse_operator                                   170       -    -170
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 1/0 up/down: 148/-170)          Total: -22 bytes
   text	   data	    bss	    dec	    hex	filename
 979938	    485	   7296	 987719	  f1247	busybox_old
 979916	    485	   7296	 987697	  f1231	busybox_unstripped

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