Don't evaluate 1-argument `ensure!` condition twice #430

Job	Run time
Test Suite (ubuntu-latest)	1m 1s
Test Suite (stable)	1m 6s
Test Suite (windows-latest)	3m 6s
Clippy (stable)	27s
Test Suite (stable, --no-default-features)	1m 0s
Check (stable)	17s
Test Suite (macOS-latest)	56s
Test Suite (stable, --no-default-features --features track-caller)	51s
Test Suite (stable, --no-default-features --features auto-install)	53s
Rustfmt (stable)	5s
Test Suite (stable, --features pyo3)	1m 3s
Miri	1m 25s
Test Suite	2m 48s
Test Suite (stable, --all-features)	1m 6s
Test Suite (--no-default-features)	2m 42s
Test Suite (beta)	1m 5s
Test Suite (--no-default-features --features track-caller)	2m 39s
Test Suite (beta, --no-default-features)	1m 1s
Test Suite (--no-default-features --features auto-install)	2m 43s
Test Suite (beta, --no-default-features --features track-caller)	1m 3s
Test Suite (beta, --no-default-features --features auto-install)	1m 2s
Test Suite (beta, --features pyo3)	1m 13s
Test Suite (beta, --all-features)	1m 12s
Test Suite (nightly)	1m 16s
Test Suite (nightly, --no-default-features)	1m 9s
Test Suite (nightly, --no-default-features --features track-caller)	1m 11s
Test Suite (nightly, --no-default-features --features auto-install)	1m 10s
Test Suite (nightly, --features pyo3)	1m 25s
Test Suite (nightly, --all-features)	1m 24s
	38m 19s

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