multiboot2: fix warning and miri error #1156

Job	Run time
build (stable) / rust	24s
build (nightly) / rust	32s
build (msrv) / rust	33s
build no_std (stable) / rust	25s
build no_std (stable) [w/o builder] / rust	16s
style (stable) / rust	9s
build no_std (stable) [Windows] / rust	2m 12s
build no_std (nightly) / rust	26s
style (nightly) / rust	17s
tests with miri (nightly) / rust	1m 43s
build no_std (msrv) / rust	23s
style (msrv) / rust	21s
	7m 41s

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