sort: fix potentially buggy use of OPT_STR

This also makes OPT_STR reused:

   text	   data	    bss	    dec	    hex	filename
 930979	    481	   6852	 938312	  e5148	busybox_old
 930954	    481	   6852	 938287	  e512f	busybox_unstripped
 ^^^^^^

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