crop: allow no-ops for 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 that.

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