Fix panic in tlsf::Heap::free #345

Job	Run time
Clippy	22s
Format	14s
check (thumbv7em-none-eabihf, nightly)	20s
test	52s
check (thumbv6m-none-eabi, stable)	25s
check (thumbv6m-none-eabi, nightly)	20s
rustdoc	18s
check (thumbv7em-none-eabihf, stable)	22s
	3m 13s

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