bc: shrink zdc_lex_string()

bc: shrink zdc_lex_string()

This actually fixes a rather obscure bug. This was failing to find
end of the string:

	$ echo -n '[foo]' | dc
	dc: string end could not be found

function                                             old     new   delta
zbc_lex_next                                        2230    2141     -89
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-89)             Total: -89 bytes
   text	   data	    bss	    dec	    hex	filename
 981461	    485	   7296	 989242	  f183a	busybox_old
 981372	    485	   7296	 989153	  f17e1	busybox_unstripped

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