der: fix append in `Encode::encode_to_vec` (#1760) #2413

Job	Run time
minimal-versions	0s
fuzz	2m 14s
test (stable)	2m 23s
build (1.81.0, wasm32-unknown-unknown)	53s
test (1.81.0)	2m 34s
build (1.81.0, thumbv7em-none-eabi)	55s
build (stable, wasm32-unknown-unknown)	1m 3s
build (stable, thumbv7em-none-eabi)	55s
	10m 57s

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