Rust wrapper: ensure memory safety for C RNG struct #6374

Job	Run time
build library (--enable-intelasm --enable-sp-asm)	31s
build library (--disable-asm --enable-mlkem --enable-lms --enable-xmss --enable-mldsa)	33s
build library (--disable-asm --enable-all-crypto --enable-mlkem --enable-lms --enable-xmss --enab...	48s
build library (--disable-asm)	36s
build library (--enable-intelasm --enable-sp-asm --enable-mlkem --enable-lms --enable-xmss --enab...	43s
build library (--enable-intelasm --enable-sp-asm --enable-all-crypto --enable-mlkem --enable-lms ...	48s
	3m 59s

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