der: pem reader rejects zero lengths reads #2307

Job	Run time
build (1.81.0, wasm32-unknown-unknown)	1m 6s
build (1.81.0, thumbv7em-none-eabi)	51s
build (stable, wasm32-unknown-unknown)	46s
build (stable, thumbv7em-none-eabi)	46s
test (1.81.0)	2m 35s
fuzz	2m 10s
test (stable)	2m 17s
minimal-versions	-1s
	10m 30s

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