Fix infinite recursion and off-by-one error in triu/tril #253

Job	Run time
docs/stable	19s
cargo-careful	1m 51s
clippy/stable	24s
cross_test/s390x-unknown-linux-gnu/stable	6m 20s
cross_test/i686-unknown-linux-gnu/stable	1m 16s
format/nightly	15s
nostd/thumbv6m-none-eabi/stable	17s
tests/stable	2m 29s
tests/beta	2m 25s
tests/nightly	2m 41s
tests/1.64.0	3m 16s
conclusion	8s
	21m 41s

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