Denial of service in HAMT Decoding in go-unixfs

go-unixfs is an implementation of a unix-like filesystem on top of an ipld merkledag. Trying to read malformed HAMT sharded directories can cause panics and virtual memory leaks. If you are reading untrusted user input, an attacker can then trigger a panic. This is caused by bogus `fanout` parameter in the HAMT directory nodes. Users are advised to upgrade to version 0.4.3 to resolve this issue. Users unable to upgrade should not feed untrusted user data to the decoding functions.