der: pem reader rejects zero lengths reads #1624

Job	Run time
build (1.85.0, wasm32-unknown-unknown)	17s
build (1.85.0, thumbv7em-none-eabi)	16s
build (stable, wasm32-unknown-unknown)	10s
build (stable, thumbv7em-none-eabi)	12s
test (stable)	21s
test (1.85.0)	28s
minimal-versions / minimal-versions	33s
	2m 17s

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