fix: reject prime == 1 in `validate_private_key_parts` (#655) #1226

Job	Run time
build (1.85.0, wasm32-unknown-unknown)	23s
test (1.85.0)	4m 19s
test (stable)	3m 56s
build (1.85.0, thumbv7em-none-eabi)	27s
nightly	1m 12s
build (stable, thumbv7em-none-eabi)	19s
build (stable, wasm32-unknown-unknown)	19s
minimal-versions	0s
	10m 55s

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