xargs: make -I imply -r

Make -I imply -r (GNU findutils seems to do the same).

Fixes the following bug:

        $ echo -n | xargs -I% echo %
        Segmentation fault

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

findutils/xargs.c

@@ -577,6 +577,9 @@ int xargs_main(int argc, char **argv)
 		G.argv = argv;
 		argc = 0;
 		read_args = process_stdin_with_replace;
+		/* Make -I imply -r. GNU findutils seems to do the same: */
+		/* (otherwise "echo -n | xargs -I% echo %" would SEGV) */
+		opt |= OPT_NO_EMPTY;
 	} else
 #endif
 	{