[9front] crop: allow no-ops in pipelines

[qwx <qwx@sciops.net>]

Hello,

Any objections to pushing this change?

Thanks,

qwx

--
crop: allow no-ops in pipelines

unlike other tools like iconv(1), a crop(1) without arguments or with
ones resulting in a no-op, like `-t 0 0', errors out.  other options
like `-i 0' do not error.  this breaks assumptions and results in
tedious intermediary steps or hacks like:

	foo | {crop -t $1 $2 >[2]/null || cat} > baz.bit

instead, just ignore the check.  subsequent code doesn't make
assumptions on it.

diff -r 936eda80a52f sys/src/cmd/crop.c
--- a/sys/src/cmd/crop.c	Sat Apr 10 15:01:09 2021 +0200
+++ b/sys/src/cmd/crop.c	Mon Apr 12 02:01:12 2021 +0200
@@ -148,9 +148,6 @@
 		usage();
 	}ARGEND
 
-	if(mode == None && cropval == 0 && eqpt(ZP, t))
-		usage();
-
 	file = "<stdin>";
 	fd = 0;
 	if(argc > 1)