der: pem reader rejects zero lengths reads #2309

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	56s
build (stable, thumbv7em-none-eabi)	50s
build (stable, wasm32-unknown-unknown)	48s
test (1.81.0)	2m 29s
build (1.81.0, wasm32-unknown-unknown)	1m 0s
fuzz	2m 9s
test (stable)	2m 22s
minimal-versions	0s
	10m 34s

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