Fix panic in tlsf::Heap::free #350

Job	Run time
rustdoc	20s
Clippy	24s
check (thumbv6m-none-eabi, stable)	24s
check (thumbv7em-none-eabihf, nightly)	21s
test	43s
Format	15s
check (thumbv7em-none-eabihf, stable)	19s
check (thumbv6m-none-eabi, nightly)	24s
	3m 10s

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