Fix the rust allocator when std is not used #394

Job	Run time
cargo-c dynamic library	1m 3s
Clippy	23s
QEMU build & test (stable, ubuntu-latest, s390x-unknown-linux-gnu, s390x-linux-gnu-gcc, qemu-s390...	42s
Build and test (stable, ubuntu-latest, x86_64-unknown-linux-gnu)	1m 2s
Clippy (--no-default-features --features="c-allocator")	9s
QEMU build & test (stable, ubuntu-latest, i686-unknown-linux-gnu, i686-linux-gnu-gcc)	22s
Build and test (msrv, ubuntu-latest, x86_64-unknown-linux-gnu)	36s
Smoke-test fuzzing targets (ubuntu-20.04, default)	1m 35s
Clippy (--no-default-features --features="std,rust-allocator")	15s
Smoke-test fuzzing targets (ubuntu-20.04, c-allocator)	1m 46s
Smoke-test fuzzing targets (ubuntu-20.04, rust-allocator)	1m 34s
vanilla dynamic library	28s
Build and test (beta, ubuntu-latest, x86_64-unknown-linux-gnu)	36s
Smoke-test fuzzing targets (macos-14)	1m 38s
Miri	1m 18s
Build and test (stable, macos-latest, x86_64-apple-darwin)	53s
Build and test (stable, macos-14, aarch64-apple-darwin)	40s
wasm32	21s
Build and test (stable-x86_64-gnu, windows-2022, x86_64-pc-windows-gnu)	1m 40s
	17m 1s

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